T4 Test

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Why is the Test Performed?

This test may be performed as part of an evaluation of thyroid function.

Thyroid function is complex, and depends on the action of many different hormones. Some hormones, such as T3, are active hormones and directly affect the way the body functions. Other hormones regulate hormone production, but do not have physiologic effects themselves.

Thyroid-stimulating hormone (TSH) is a hormone secreted by the pituitary gland. It stimulates the synthesis and secretion of T4 (thyroxine) and T3 (triiodothyronine), by the thyroid gland. TSH is stimulated by another hormone, thyroid-releasing hormone (TRH), which is released by the hypothalamus.

T3 and T4 feedback inhibit the release of both TSH and TRH in people with normal thyroid function. Most of the thyroid hormone secreted by the thyroid gland is in the form of T4, but T3 is the active hormone; i.e., T4 is converted to T3 by tissues in the body.

T4 is the major hormone controlling the basal metabolic rate. The exact mechanisms are not completely known, but it is known that T4 increases the concentrations of numerous enzymes involved in the production of energy in all nucleated cells of the body. Most of the T4 in the blood is bound to proteins; this acts as a reservoir of available thyroxine because only free T4 is active on cells. T4 appears to be converted to T3 within cells, before it enters the nucleus and interacts direction with DNA, eventually resulting in the production of various proteins by the cell.

Only 0.03% of the T4 in plasma is free (soluble and active) and 0.3% of the T3 is free; the rest is bound to protein. Most of the T3 and T4 are transported by TBG (thyroxine binding globulin), but smaller amounts are found on prealbumin and albumin. The available concentrations of T3 and T4 are affected by the level of TBG. Certain conditions can change the level of TBG in the body, such as estrogen.