Clinical Information

Thyroxine (T4) and triiodothyronine (T3) are the 2 biologically active thyroid hormones. T4 accounts for more than 80% of circulating thyroid hormone.

After secretion by the thyroid gland, approximately 70% of circulating T4 and T3 bind to thyroid-binding globulin (TBG), and 10% to 20% bind to transthyretin (TTR) and albumin, respectively. Less than 0.1% circulates as free T4 (FT4) or free T3 (FT3). FT4 and FT3 enter and leave cells freely by diffusion. Only free hormones are biologically active; however, the bound and free fractions remain in equilibrium. Equilibrium with TTR and albumin occurs rapidly, whereas, TBG binds thyroid hormones very tightly and equilibrium dissociation is slow. TBG-bound thyroid hormone serves as a hormone reservoir, and T4 serves as a prohormone for T3. Within cells, T4 is either converted to T3, which is about 5 times as potent as T4, or reverse T3, which is biologically inactive. Ultimately, T3, and to a much lesser degree T4, bind to nuclear thyroid hormone receptors, altering gene expression in a tissue-specific manner.

Under normal physiologic conditions, FT4 and FT3 exert direct and indirect negative feedback on pituitary thyrotropin (TSH), the major hormone regulating thyroid gland activity. This feedback tightly regulates thyroid hormone production and maintains stable FT4 and FT3 concentrations independent of binding protein concentrations. Therefore. measurement of FT4 and FT3 in conjunction with TSH provides the best assessment of thyroid function. It also allows determination of whether hyperthyroidism (increased FT4) or hypothyroidism (low FT4) are primary (most cases; TSH altered in the opposite direction as FT4) or secondary/tertiary (hypothalamic/pituitary origin; TSH altered in the same direction as FT4). In contrast, total T4 and T3 levels may vary widely as a response to changes in binding protein levels, without any change in free thyroid hormone levels and, hence, actual thyroid function status.

Free T4 is usually measured using automated analog immunoassays, which  provide accurate results in most cases. However, abnormal types or quantities of binding proteins found in some patients and most often related to other illnesses or drug treatments, may interfere in the accurate measurement of FT4 by analog immunoassays. These problems can be overcome by measuring FT4 by equilibrium dialysis, free from interfering proteins.