Thyroid Problems Overview
The thyroid gland is located on the front part of the neck below the thyroid cartilage (Adam's apple). The gland produces thyroid hormones, which regulate metabolic rate (how fast calories are consumed to produce energy). Thyroid hormones are important in regulating body energy, body temperature, the body's use of other hormones and vitamins, and the growth and maturation of body tissues.
Diseases of the thyroid gland can result in either production of too much (overactive
thyroid disease or hyperthyroidism), too little (underactive
thyroid disease or hypothyroidism)
thyroid hormone, thyroid nodules, and/or goiter.
are much more common in women than in men.
- Production of thyroid hormones: The process of hormone synthesis begins in a part of the brain called the hypothalamus. The hypothalamus releases thyrotropin-releasing hormone (TRH). The
TRH travels through the venous plexus located in the pituitary stalk to the pituitary gland, also in the brain. In response, the pituitary gland then releases thyroid-stimulating hormone (TSH,
also called thyrotropin) into
the blood. The TSH travels to the thyroid gland and stimulates the thyroid to produce the two thyroid hormones, L-thyroxine (T4) and triiodothyronine (T3). The thyroid gland also needs adequate
amounts of dietary iodine to be able to produce T4 and T3, the molecules of
which contain four and three atoms of iodine, respectively.
- Regulation of thyroid hormone
production: To prevent the overproduction or underproduction of thyroid hormones, the pituitary gland senses how much hormone is in the blood and adjust the production of hormones
accordingly. For example, when there is too much thyroid hormone in the blood, TRH and TSH production are both decreased. The sum effect of this is to decrease the amount of TSH released from the
pituitary gland and to reduce production of thyroid hormones from the thyroid gland to restore the amount of thyroid hormone in the blood to normal. Defects in these regulatory pathways
rarely may result
in hypothyroidism (underactive thyroid problem) or hyperthyroidism
(overactive thyroid problem). The most common cause of hypothyroidism and
hyperthyroidism occurs due to problems within the thyroid and not the regulatory
- Thyroid goiter: Thyroid goiter is any enlargement of the thyroid that
can occur with hyperthyroidism or hypothyroidism but also with benign and
malignant (cancerous) nodules. Worldwide, the most common cause of goiter is
iodine deficiency. Although it used to be very common in the U.S., it is now
less common with the use of iodized salt. Multiple nodules in the thyroid are
very common, but only about 5% of the nodules are a thyroid cancer. Thyroid cancer rates have been increasing steadily by about 6% every year for more than 20 years. It is one of the few cancers whose rate is increasing and whose very low rate of mortality is also rising with time. Although radiation exposure as a child can increase the risk of thyroid cancer, we do not know why the overall rate has been increasing. Thyroid
cancer is diagnosed after a thyroid ultrasound exam and a needle aspiration
biopsy of the nodule.
Stephanie L Lee, MD, PhD, FACE
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