In 1992, researchers at the National Cancer Institute (NCI) initiated a study on the long-term health effects associated with silicone breast implants. It is estimated that between 1.5 million and 2 million women in this country have had breast implants since they first appeared on the market in 1962.

One of the longest and largest studies to date on the health effects of implants, the NCI report involves 13,500 women with an average follow-up time of 13 years. Most previous investigations have looked at the health effects over a shorter time period, typically less than 10 years, and have been too small to evaluate uncommon diseases.

In addition, earlier reports did not include detailed information about types of implants or risk factors affecting health, such as medical history, screening practices, and lifestyle behaviors all of which are included in the current study.

Another unique feature of the NCI study is that the investigators compared the health risks of the implant patients to both the general population and other plastic surgery patients. Prior reports have used only the general population as a comparison group.

The purpose of the NCI study is to evaluate the effect of implants on the risk of:

• Developing breast cancer;
• Developing cancers other than breast;
• Dying from all causes of mortality; and
• Developing connective tissue disorders.

The results will be published in various scientific journals. As the publications appear in the literature, the findings will be summarized at the end of this fact sheet.

Background

Breast implants were first marketed in the early 1960s, before the 1976 Medical Device Amendments to the Food, Drug and Cosmetic Act required that medical devices be shown to be safe and effective. Silicone was initially assumed by manufacturers to be biologically inactive and, therefore, to have no harmful effects. However, cases of connective tissue disorders and cancers were reported in several short-term studies.

Because there were few reports on the long-term safety of implants, in 1992, the Food and Drug Administration (FDA) restricted the use of silicone gel breast implants to controlled clinical trials of women seeking breast reconstruction. That year, Congress also directed the National Institutes of Health to undertake a large follow-up study to assess the long-term health effects of exposure to silicone breast implants.

Types of Implants

Implants are soft silicone sacs or shells, inflated with either saline solution (salt water) or a synthetic silicone gel. Until the FDA ban in 1992, 90 percent to 95 percent of the implants contained the silicone gel because it had a more pleasing look and feel than the saline-filled implants. Since the 1992 ban, 90 percent to 95 percent of the implants have been saline-filled. Currently, it is not known how many women have silicone vs. saline implants, but women with both types are included in the NCI study.

Previous Studies

About 80 percent of breast implants in the United States are for cosmetic reasons and 20 percent for breast reconstruction after breast cancer surgery. The majority of previous studies have focused on women who received implants for cosmetic reasons.

1. Breast Cancer Risk

   A number of previous studies have evaluated the relationship between breast implants and subsequent breast cancer risk. Most have shown that the risk of developing breast cancer is less among women with implants compared to women without implants. In several of the studies, the size of the reduced risk was as much as 50 percent to 60 percent. However, the vast majority did not have enough detailed information on patient characteristics that could affect the development of breast cancer, and had follow-up times of less than 10 years.

2. Stage at Diagnosis of Breast Cancer

   Some clinical studies have suggested that women with breast implants have more advanced breast cancer at diagnosis than women without breast implants. This is because implants have been reported to decrease the ability to detect breast lesions, with either clinical examination or mammography. However, in two larger epidemiologic studies, there did not appear to be differences in stage at diagnosis or survival rates among the two groups.

3. Mortality

   No prior studies have evaluated all causes of mortality of breast implant patients but, instead, have limited their analyses to mortality from breast cancer. Compared to the general population, no increased risk in breast cancer mortality for implant patients was observed.

4. Types of Implant

   Because earlier reports did not include detailed information about the types of implants, an evaluation of the effect of the implant type on the health risks of the patients has not been possible.

5. Connective Tissue Disorders

   Anecdotal reports have suggested increased risks of certain connective tissue disorders, including scleroderma, systemic lupus erythematosus, rheumatoid arthritis, and Sjogren's syndrome. The size of the studies, however, has not been large enough to draw any definite conclusions.

6. Women who receive implants for breast reconstructive surgery

   One small study reported no increase in the development of second primary breast cancer in women with silicone implants following mastectomy compared to women who received mastectomies without implants. The small size of the study, however, limits the conclusions.

   Note: Any study of the risks of breast cancer or other cancers with women who receive reconstructive implants is more complicated than one involving women with cosmetic implants because it needs to take into account the effects of different breast cancer treatments. A study with breast cancer patients would best be done in the context of a clinical trial where comparisons can be made between women who choose to have reconstruction and those who do not, but who otherwise have received identical treatments.

Patient Population in the NCI Study

The participants include 13,500 women who had implant surgery for cosmetic reasons in both breasts before 1989. For comparison, about 4,000 women similar in age who had some other type of plastic surgery, such as removal of fat from the stomach or wrinkles from the face or neck, were identified. All participants were from 18 plastic surgery practices in six geographic areas (Atlanta, Ga.; Birmingham, Ala.; Charlotte, N.C.; Miami and Orlando, Fla.; and Washington, D.C.). The practices were chosen because the plastic surgeons had performed large numbers of cosmetic breast implant surgeries prior to 1989 and were willing to give the investigators access to their records. The health effects of the implant patients were also compared to the general population.

Half (49.7 percent) of the participants received silicone gel implants, 34.1 percent double lumen implants, (these have two shells; the inner sac is filled with silicone gel and the outer one with saline), 12.2 percent saline-filled implants, 0.1 percent other types of implants, and 3.8 percent unspecified types of implants.

The study participants had cosmetic surgery between 1962 and 1988 during a time when a great number of changes were taking place in the manufacturing of breast implants such as the shell thickness, the type of shell coating, and the silicone gel composition. There were no women included in the study who received implants following a diagnosis of breast cancer.

Study Design

The medical records from the plastic surgery practices were reviewed to identify patients who were eligible for the study. For eligible patients, trained medical records abstractors collected information about the surgical procedures, the type of implant, any complications, and factors which might affect health status, such as weight or medical history.

Patients were then traced through a variety of sources. Living subjects were asked to complete a mailed questionnaire to collect information about their health status, including whether they had subsequent plastic surgery, as well as lifestyle factors that could affect their health (menstrual, pregnancy, and breast-feeding history, weight, hormone use, cigarette smoking, alcohol consumption, and medical history). Extensive data on the potential short-term (rupture) and longer-term complications (cancer, connective tissue diseases, symptoms of connective tissue disease) were also obtained through the questionnaire.

No clinical examinations were done on the living patients for this study. Attempts were made to verify patient reports of cancer and connective tissue diseases from the medical records of physicians who had diagnosed or treated these diseases. To verify the causes of death, death certificates were collected for the patients who had died. About 80 percent of the original 13,500 implant patients and 4,000 controls were successfully traced. About 70 percent of those traced as alive completed the questionnaires. These percentages are similar to other comparably designed epidemiologic studies.

Special Issues

Because of the highly controversial and political nature of the study, maintaining objectivity was a particularly important issue. The following steps were taken to ensure scientific objectivity:

• The study was funded entirely by the government, not by plastic surgeons, implant manufacturers, or other special interest groups.

• Government scientists, not manufacturers of implants or plastic surgeons, designed and provided scientific oversight for the study.

• To participate in the study, plastic surgeons had to agree to allow investigators to see all of their records, not just records for selected patients. The investigators obtained detailed information on factors which could affect health status.

• Information gathered through the participant questionnaire about diseases such as cancer or connective tissue disorders was confirmed by the medical records of oncologists, rheumatologists, and other physicians who had seen the study respondents for these illnesses.

• Death certificates were collected for the patients identified as deceased to verify the causes of death.

• Scientific oversight for the study has been provided by NCI's Board of Scientific Counselors and its Breast Implant Study Advisory Panel, a multidisciplinary panel of academic scientists, including oncologists, plastic surgeons, rheumatologists, and epidemiologists, as well as patient advocates and consumer representatives. The panel meets regularly with NCI researchers to address issues involving data analysis and interpretation, and reports to NCI's Board of Scientific Counselors.

• A number of professional plastic surgery organizations, including the American Society of Plastic and Reconstructive Surgeons and the American Aesthetic Society, were consulted to help identify long-standing practices in which at least 500 cosmetic operations were done prior to 1989 and in which the surgeons retained complete records and were willing to give the researchers complete access to them. The NCI investigators performed their own review of the patient records.

Investigators

Louise A. Brinton, Ph.D., is chief of the Environmental Epidemiology Branch in NCI's Division of Cancer Epidemiology and Genetics (DCEG). Additional NCI investigators are Jay H. Lubin, Ph.D., and Robert N. Hoover, M.D., also from DCEG. S. Lori Brown, Ph.D., is a research scientist from the FDA's Center for Devices and Radiological Health in Rockville, Md. Theodore Colton, Sc.D., from the Department of Epidemiology and Biostatistics at the Boston University School of Public Health in Massachusetts, was under contract with the study team, as was Mary Cay Burich from Abt Associates Inc., in Chicago, who assisted with specific data collection tasks.

Results/Publications

• The authors showed that other plastic surgery patients are a more appropriate comparison group than women in the general population for studies of the health effects of breast implants.

In analyzing the data from the 7,447 breast implant patients and 2,203 patients with other types of plastic surgery, there were no differences between the two groups with respect to family income, number of pregnancies, alcohol consumption, cigarette smoking, histories of previous gynecologic operations, or operations for benign breast disease.

Reference: Brinton LA, Brown SL, Colton T, et al. Characteristics of a Population of Women with Breast Implants Compared with Women Seeking Other types of Plastic Surgery. Plastic and Reconstructive Surgery 2000;105(3):919–27.

• The NCI researchers found no association between breast implants and the subsequent risk of breast cancer.

In addition, they found no link between breast cancer risk and number of years of follow-up, nor with any particular type of implant. They did, however, see a shift toward somewhat later detection of breast cancers among the implant patients compared to the controls. Even though the differences were not statistically significant, there were consistently smaller percentages of in situ (early-stage) cancers among the implant patients. However, there was no significant difference in breast cancer mortality between the implant and comparison patients.

Reference: Brinton LA, Lubin JH, Burich MC, et al. Breast Cancer Following Augmentation Mammoplasty (United States). Cancer Causes & Control 2000; 11(9):819–827.

• Women with implants were not at increased risk for most cancers with the exception of respiratory and brain cancers. The significance of the increased risks is not clear.

NCI researchers did not find increased risks for cancers of the stomach, large intestine, cervix, uterus, ovary, bladder or thyroid. Likewise, neither connective tissue nor immune system cancers such as soft tissue sarcomas, lymphoma, and multiple myeloma, developed at higher rates. These cancers were previously linked to implants in smaller studies. The cancer rates for brain and respiratory cancers, however, were two to three times greater in the implant patients compared to other plastic surgery patients; only the rates of respiratory cancers reached statistical significance. The significance of the findings is not clear. It is possible that the higher risks observed for respiratory and brain cancers are not related to exposure to silicone, but are due to either chance findings or to factors common to women who choose to have implants.

Reference: Brinton LA, Lubin JH, Burich MC, et al. Cancer Risk at Sites Other than Breast Following Augmentation Mammoplasty. Annals of Epidemiology May 2001;11(4):248–256.

• Women with implants were not at increased risk for most causes of death compared to the control group. The exceptions were deaths from suicide, and brain and respiratory cancers, but the significance of these increased risks is not clear.

The researchers found that nearly every cause of death, including all cancers, circulatory and digestive system diseases, and endocrine, nutritional, metabolic and immune diseases, was decreased among implant patients compared to the general population. However, implant patients were three times more likely to die from respiratory tract cancer, two to three times more likely to die from brain cancer, and four to five times more likely to die from suicide than the comparison group; only the rates for respiratory cancers reached statistical significance. It is possible that these higher risks are not related to exposure to silicone, but are due to either chance findings or to factors common to women who choose to have implants.

Reference: Brinton LA, Lubin JH, Burich MC, et al. Mortality Among Augmentation Mammoplasty Patients. Epidemiology May 2001;12(3):321–326.

• Women with breast implants were not at an increased risk for connective tissue disorders when their records were reviewed by two independent rheumatologists who did not know if the women had an implant or not.

According to initial reporting by an implant patient questionnaire, there appeared to be a two fold increased risk for developing rheumatoid arthritis, scleroderma, systemic lupus erythematosus, and Sjogren's syndrome in this population. NCI researchers were able to access only 34–40% of these patients' records who reported being diagnosed with a connective tissue disorder. When these records were reviewed by two independent, board-certified rheumatologists who did not know if the patient had an implant or not, the rheumatologists found between 17–30% of the diagnoses likely. As a result, the risk for developing a connective tissue disorder due to breast implants, while still somewhat elevated, became statistically non-significant. Further research including records linkage, standardized diagnostic criteria for rheumatoid disorders, and clinical exams, is needed to understand if a relationship exists.

Reference: Brinton LA, Buckley LM, Dvorkina O, et al. Risk of Connective Tissue Disorders among Breast Implant Patients. American Journal of Epidemiology; 2004;160; 619–627.

• In an extended mortality follow-up study, a number of the previously observed excesses were attenuated. The risk of suicides continued to be of concern; in addition, a new excess risk among implant patients of deaths from motor vehicle accidents emerged.

Participants in this study were followed for an additional 5 years to further examine mortality rates. Breast implant participants, in comparison to patients with other types of plastic surgery, were 1.6 times as likely to die from respiratory cancer, two times as likely to die from brain cancer, and 2.6 times as likely to die from suicide. No additional deaths from brain cancers were found during the extended follow-up period. New findings include the excess risk of death (1.7 times as likely) from motor vehicle accidents. The researchers suggest that some of the accidents may not have been totally accidental, or may have reflected alcohol/drug dependencies since higher rates of death due to these dependencies were also observed among implant patients. It is possible that the elevated mortality risks observed during this extended followup are not related to exposure to silicone, but are due to either chance findings or to factors common to women who choose to have implants.

Reference: Brinton LA, Lubin JH, Murray MC, et al. Mortality Rates Among Augmentation Mammoplasty Patients: An Update. Epidemiology March 2006; 17(2): 162-169.

Related Links:

NCI news Web site for press releases about breast implants: Oct. 2, 2000 for breast implants and breast cancer risk; April 23, 2001 for press releases on risk of other cancers; April 27, 2001 for causes of mortality release. http://www.cancer.gov

FDA Web site on breast implants: http://www.fda.gov/cdrh/breastimplants/.

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• Cancer Causes and Risk Factors Home Page ² (http://www.cancer.gov/cancertopics/prevention-genetics-causes/causes)
• What You Need To Know About™ Breast Cancer ³
  (http://www.cancer.gov/cancertopics/wyntk/breast)
• What You Need To Know About™ Cancer ⁴
  (http://www.cancer.gov/cancertopics/wyntk/overview)

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Glossary Terms

A chemical substance found in beer, wine, and liquor. Alcohol is also found in some medicines, mouthwashes, essential oils (scented liquid taken from plants), and household products.

A process in which anything complex is separated into simple or less complex parts.

An incomplete description of the medical and treatment history of one or more patients. Anecdotal reports may be published in places other than peer-reviewed, scientific journals.

Weakened or thinned. Attenuated strains of disease-causing bacteria and viruses are often used as vaccines. The weakened strains are used as vaccines because they stimulate a protective immune response while causing no disease or only mild disease in the person receiving the vaccine.

A common condition marked by benign (not cancer) changes in breast tissue. These changes may include irregular lumps or cysts, breast discomfort, sensitive nipples, and itching. These symptoms may change throughout the menstrual cycle and usually stop after menopause. Also called fibrocystic breast changes, fibrocystic breast disease, and mammary dysplasia.

The science of collecting and analyzing biologic or health data using statistical methods. Biostatistics may be used to help learn the possible causes of a cancer or how often a cancer occurs in a certain group of people. Also called biometrics and biometry.

The organ that stores urine.

Cancer that forms in tissues of the breast, usually the ducts (tubes that carry milk to the nipple) and lobules (glands that make milk). It occurs in both men and women, although male breast cancer is rare.

A silicone gel-filled or saline-filled sac placed under the chest muscle to restore breast shape.

Surgery to rebuild the shape of the breast after a mastectomy.

A term for diseases in which abnormal cells divide without control and can invade nearby tissues. Cancer cells can also spread to other parts of the body through the blood and lymph systems. There are several main types of cancer. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Sarcoma is a cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue. Leukemia is a cancer that starts in blood-forming tissue such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the blood. Lymphoma and multiple myeloma are cancers that begin in the cells of the immune system. Central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord. Also called malignancy.

The lower, narrow end of the uterus that forms a canal between the uterus and vagina.

A type of research study that tests how well new medical approaches work in people. These studies test new methods of screening, prevention, diagnosis, or treatment of a disease. Also called clinical trial.

In medicine, a medical problem that occurs during a disease, or after a procedure or treatment. The complication may be caused by the disease, procedure, or treatment or may be unrelated to them.

Supporting tissue that surrounds other tissues and organs. Specialized connective tissue includes bone, cartilage, blood, and fat.

In a clinical trial, the group that does not receive the new treatment being studied. This group is compared to the group that receives the new treatment, to see if the new treatment works.

A clinical study that includes a comparison (control) group. The comparison group receives a placebo, another treatment, or no treatment at all.

The process of identifying a disease, such as cancer, from its signs and symptoms.

The organs that take in food and turn it into products that the body can use to stay healthy. Waste products the body cannot use leave the body through bowel movements. The digestive system includes the salivary glands, mouth, esophagus, stomach, liver, pancreas, gallbladder, small and large intestines, and rectum.

In medicine, a disturbance of normal functioning of the mind or body. Disorders may be caused by genetic factors, disease, or trauma.

Refers to tissue that makes and releases hormones that travel in the bloodstream and control the actions of other cells or organs. Some examples of endocrine tissues are the pituitary, thyroid, and adrenal glands.

The study of the patterns, causes, and control of disease in groups of people.

An agency in the U.S. federal government whose mission is to protect public health by making sure that food, cosmetics, and nutritional supplements are safe to use and truthfully labeled. The FDA also makes sure that drugs, medical devices, and equipment are safe and effective, and that blood for transfusions and transplant tissue are safe. Also called Food and Drug Administration.

An agency in the U.S. federal government whose mission is to protect public health by making sure that food, cosmetics, and nutritional supplements are safe to use and truthfully labeled. The Food and Drug Administration also makes sure that drugs, medical devices, and equipment are safe and effective, and that blood for transfusions and transplant tissue are safe. Also called FDA.

The study of genes and heredity. Heredity is the passing of genetic information and traits (such as eye color and an increased chance of getting a certain disease) from parents to offspring.

Having to do with the female reproductive tract (including the cervix, endometrium, fallopian tubes, ovaries, uterus, and vagina).

One of many chemicals made by glands in the body. Hormones circulate in the bloodstream and control the actions of certain cells or organs. Some hormones can also be made in the laboratory.

The complex group of organs and cells that defends the body against infections and other diseases.

The long, tube-like organ that is connected to the small intestine at one end and the anus at the other. The large intestine has four parts: cecum, colon, rectum, and anal canal. Partly digested food moves through the cecum into the colon, where water and some nutrients and electrolytes are removed. The remaining material, solid waste called stool, moves through the colon, is stored in the rectum, and leaves the body through the anal canal and anus.

An area of abnormal tissue. A lesion may be benign (not cancer) or malignant (cancer).

Cancer that begins in cells of the immune system. There are two basic categories of lymphomas. One kind is Hodgkin lymphoma, which is marked by the presence of a type of cell called the Reed-Sternberg cell. The other category is non-Hodgkin lymphomas, which includes a large, diverse group of cancers of immune system cells. Non-Hodgkin lymphomas can be further divided into cancers that have an indolent (slow-growing) course and those that have an aggressive (fast-growing) course. These subtypes behave and respond to treatment differently. Both Hodgkin and non-Hodgkin lymphomas can occur in children and adults, and prognosis and treatment depend on the stage and the type of cancer.

The use of film or a computer to create a picture of the breast.

Surgery to remove the breast (or as much of the breast tissue as possible).

An instrument, tool, machine, test kit, or implant that is used to prevent, diagnose, or treat disease or other conditions. Medical devices range from tongue depressors to heart pacemakers and medical imaging equipment.

The monthly cycle of hormonal changes from the beginning of one menstrual period to the beginning of the next.

Having to do with metabolism (the total of all chemical changes that take place in a cell or an organism to produce energy and basic materials needed for important life processes).

The state of being mortal (destined to die). Mortality also refers to the death rate, or the number of deaths in a certain group of people in a certain period of time. Mortality may be reported for people who have a certain disease, live in one area of the country, or who are of a certain gender, age, or ethnic group.

A type of cancer that begins in plasma cells (white blood cells that produce antibodies). Also called Kahler disease, myelomatosis, and plasma cell myeloma.

The National Cancer Institute, part of the National Institutes of Health of the United States Department of Health and Human Services, is the Federal Government's principal agency for cancer research. The National Cancer Institute conducts, coordinates, and funds cancer research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer. Access the National Cancer Institute Web site at http://www.cancer.gov. Also called NCI.

A federal agency in the U.S. that conducts biomedical research in its own laboratories; supports the research of non-Federal scientists in universities, medical schools, hospitals, and research institutions throughout the country and abroad; helps in the training of research investigators; and fosters communication of medical information. Access the National Institutes of Health Web site at http://www.nih.gov. Also called NIH.

NCI, part of the National Institutes of Health of the United States Department of Health and Human Services, is the Federal Government's principal agency for cancer research. It conducts, coordinates, and funds cancer research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer. Access the NCI Web site at http://www.cancer.gov. Also called National Cancer Institute.

The taking in and use of food and other nourishing material by the body. Nutrition is a 3-part process. First, food or drink is consumed. Second, the body breaks down the food or drink into nutrients. Third, the nutrients travel through the bloodstream to different parts of the body where they are used as "fuel" and for many other purposes. To give the body proper nutrition, a person has to eat and drink enough of the foods that contain key nutrients.

One of a pair of female reproductive glands in which the ova, or eggs, are formed. The ovaries are located in the pelvis, one on each side of the uterus.

Medical doctor.

An operation that restores or improves the appearance of body structures.

The organs that are involved in breathing. These include the nose, throat, larynx, trachea, bronchi, and lungs. Also called respiratory system.

An autoimmune disease that causes pain, swelling, and stiffness in the joints, and may cause severe joint damage, loss of function, and disability. The disease may last from months to a lifetime, and symptoms may improve and worsen over time.

Something that increases the chance of developing a disease. Some examples of risk factors for cancer are age, a family history of certain cancers, use of tobacco products, being exposed to radiation or certain chemicals, infection with certain viruses or bacteria, and certain genetic changes.

A solution of salt and water.

A chronic disorder marked by hardening and thickening of the skin. Scleroderma can be localized or it can affect the entire body (systemic).

Checking for disease when there are no symptoms. Since screening may find diseases at an early stage, there may be a better chance of curing the disease. Examples of cancer screening tests are the mammogram (breast), colonoscopy (colon), Pap smear (cervix), and PSA blood level and digital rectal exam (prostate). Screening can also include checking for a person’s risk of developing an inherited disease by doing a genetic test.

A synthetic gel that is used as an outer coating on breast implants and as the inside filling of some implants.

A cancer that begins in the muscle, fat, fibrous tissue, blood vessels, or other supporting tissue of the body.

The extent of a cancer in the body. Staging is usually based on the size of the tumor, whether lymph nodes contain cancer, and whether the cancer has spread from the original site to other parts of the body.

Describes a mathematical measure of difference between groups. The difference is said to be statistically significant if it is greater than what might be expected to happen by chance alone. Also called significant.

An organ that is part of the digestive system. The stomach helps digest food by mixing it with digestive juices and churning it into a thin liquid.

The act of taking one's own life on purpose.

The percentage of people in a study or treatment group who are alive for a certain period of time after they were diagnosed with or treated for a disease, such as cancer. The survival rate is often stated as a five-year survival rate, which is the percentage of people in a study or treatment group who are alive five years after diagnosis or treatment. Also called overall survival rate.

An indication that a person has a condition or disease. Some examples of symptoms are headache, fever, fatigue, nausea, vomiting, and pain.

A set of symptoms or conditions that occur together and suggest the presence of a certain disease or an increased chance of developing the disease.

Having to do with substances that are man-made instead of taken from nature.

A chronic, inflammatory, connective tissue disease that can affect the joints and many organs, including the skin, heart, lungs, kidneys, and nervous system. It can cause many different symptoms; however, not everyone with systemic lupus erythematosus has all of the symptoms. Also called lupus and SLE.

A gland located beneath the larynx (voice box) that makes thyroid hormone and calcitonin. The thyroid helps regulate growth and metabolism. Also called thyroid gland.

The small, hollow, pear-shaped organ in a woman's pelvis. This is the organ in which a fetus develops. Also called womb.