Thyroid Physiology

The thyroid gland is the biggest gland in the neck. It is situated in the front of the neck, just below your Adam’s apple, on either side of the neck, and partly wraps around the trachea (wind pipe). The sole function of the thyroid is to make thyroid hormone. This hormone has an effect on nearly all tissues of the body where it increases cellular activity. The function of the thyroid therefore is to regulate the body's metabolism.

The thyroid gland receives much if its instruction from the pituitary gland, a small gland located at the base of brain. The pituitary gland secretes Thyroid Stimulating Hormone (TSH). TSH, as its name implies, stimulates the thyroid gland to start producing thyroid hormone. When the thyroid gland starts to slow down or become underactive, it doesn’t use up all the TSH being released by the pituitary, and the TSH levels start to rise.

TSH blood levels are commonly measured as a way to screen for thyroid abnormalities. The normal range for most labs is 0.5 – 5.5. The higher the number, the greater the indication of underactive thyroid. At Valley Sports Physicians, we prefer to see people in the “optimum” range, under 2.0.

The main hormone produced by the thyroid gland is T4, also known as levothyroxine. It is a molecule that contains 4 iodine units (hence the “T4”). The T4 gets secreted by the thyroid gland and enters the blood stream. Once it reaches the body, T4 gets converted to T3 (tri-iodothyronine) by cleaving off 1 iodine unit. (A small amount of T3 is also produced directly in the body.) T3 is 4-5 times more active or powerful than T4 and is the real thyroid hormone.

If the thyroid gland is underactive (hypothyroidism), insufficient amounts of T4 are produced, resulting in too little T3. On the other hand, your body may also not adequately convert T4 to T3, in spite of adequate T4 levels.

There are several blood tests to measure T3 and T4 levels. At Valley Sports Physicians, we find the most useful tests to be the Free T3 and Free T4 levels. These test measure the amount of T3 and T4 that is “free” in the blood to be used by the body. We maintain that optimum levels of T3 should be over 4 pg/ml (the normal range is 2.3-4.2).

The most common thyroid medications for treating hypothyroidism include Synthroid® and Levoxyl®, brand names for levothyroxine (T4). Armour Thyroid® is also used; this is a combination of T4 and T3. It is a natural product derived from pig thyroid. A synthetic form is also available known as Thyrolar®. We prefer that brand names be requested, in order to insure consistent quality and bioavailability. The added cost is minimal.

Thyroid blood tests should be repeated in about 6 weeks after either starting treatment or changing doses.

For more information, go to www.endocrineweb.com/thyfunction.html.