T

testosterone A steroid HORMONEthe adrenal cor-
tex of the ADRENAL GLANDS, the TESTESin men, and
the OVARIES in women synthesize from a base
ingredient of cholesterol. Adipose (fat) cells
throughout the body also produce small amounts
of testosterone. Testosterone is one of the ANDRO-
GENSand the predominant male sex hormone. It is
responsible for male secondary sex characteristics,
male FERTILITY, and spermatogenesis (SPERM pro-
duction). In men and women both testosterone is
important for MUSCLE mass, BONE DENSITY, and
LIBIDO(sex drive).
In men testosterone levels peak around age 22,
then decline at the rate of about 10 percent per
decade until about age 75. Changes in a man’s
body shape begin to take place when the testos-
terone level reaches about 60 percent of its peak
level, when a man is in his late 50s and early 60s.
These changes include diminishing muscle mass,
increased and redistributed body fat, loss of the
HAIR on the head, and slower sexual response.
Some men equate these midlife changes with
“male MENOPAUSE” or ANDROPAUSE. In women
testosterone levels cyclically fluctuate with the
MENSTRUATIONuntil menopause, after which the
levels of testosterone and ESTROGENSdrop signifi-
cantly. Some women experience diminished sex-
ual response as a result.
Endocrinologists may prescribe low-DOSEtestos-
terone supplement to restore sexual response in
men and women. Testosterone supplement also
enhances spermatogenesis in men and may be a
treatment for male INFERTILITY.
For further discussion of testosterone within
the context of the endocrine system’s structure
and function please see the overview section “The
Endocrine System.”

See also ALOPECIA; ANABOLIC STEROIDS AND STEROID

PRECURSORS; HIRSUTISM; HYPOGONADISM; INHIBIN; PRO-

GESTERONE.

thymosin A peptide HORMONE the THYMUS pro-

duces that influences how and when T-cell lym-

phocytes (white BLOODcells that fight INFECTION)

mature. The epithelial cells of the outer structure

of the thymus synthesize thymosin most actively

during childhood. Researchers do not fully under-

stand the functions of the thymus or thymosin,

particularly in adulthood. Current research is

exploring the potential for using thymosin supple-

ment to treat diseases such as HEPATITIS C and

HIV/AIDS.

For further discussion of thymosin within the

context of the endocrine system’s structure and

function please see the overview section “The

Endocrine System.”

See also IMMUNODEFICIENCY; LYMPHOCYTE.

thyroid cancer Malignant growths that develop

within the tissues of the THYROID GLAND. Thyroid

CANCERmay be primary (originating in the thyroid

gland) or secondary (metastasizing from cancer

that originates elsewhere in the body). Thyroid

cancer is uncommon in the United States, with

about 11,000 cases diagnosed each year, and

occurs primarily in people who are over age 70.

There are four kinds of thyroid cancer: papillary,

follicular, medullary, and anaplastic.

Papillary thyroid cancer About 75 percent of

people who have thyroid cancer have papillary

CARCINOMA, which is highly curable when detected

and removed while the tumor is still encapsulated

and clearly defined. Papillary thyroid cancer gen-

erally begins as a painless, single lump (nodule)

160