T3 is measured as part of a thyroid function evaluation. T3 may be measured in cases in which there is some doubt about whether the patient has hyperthyroidism or hypothyroidism after measuring T4 and RT3U. For example, in some cases, symptoms of hyperthyroidism are apparent but T4 is normal and T3 is elevated.

TSH (thyroid-stimulating hormone) is a peptide hormone secreted by the pituitary gland. It stimulates the synthesis and secretion of T4 (thyroxine) and T3 from the thyroid gland. TSH is, itself, stimulated by TRH (thyrotropin-releasing hormone), which is released by the hypothalamus. T3 and T4 feedback inhibit the release of both TSH and TRH in normal people. Most of the thyroid hormone secreted by the thyroid gland is in the form of T4, but T3 is probably the active hormone; that is, T4 is converted to T3 by target tissues.

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 since only free T4 is active in cells. Only 0.03% of the T4 in plasma is free (soluble and active) and 0.3% of the T3 is free. 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, which is measured by the R3TU test. 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.

• hyperthyroidism (e.g., Graves' disease)
• T3 thyrotoxicosis (rare)
• thyroiditis
• thyroid cancer (rare)

• chronic illness
• hypothyroidism (for example, Hashimoto's disease)
• starvation

• painless (silent) thyroiditis
• thyrotoxic periodic paralysis
• toxic nodular goiter