A challenge of several concepts of free thyroxin index for assessing thyroid status in patients with altered thyroid-binding protein capacity.

I compared values for the free thyroxin index derived from data obtained by four triiodothyronine-uptake kits for 93 euthyroid subjects with normal or altered thyroxin-binding globulin binding capacity. In addition, I devised a mathematical model of an "ideal" free thyroxin index. Only one uptake kit satisfactorily normalized values for the free thyroxin index in the increased thyroglobulin groups, as compared with the control. The other three demonstrated contrasting patterns of free thyroxin index variation as compared with normal, when thyroglobulin concentrations were increased. By all four uptake tests, values for the free thyroxin index in thyroglobulin-deficient patients were significantly lower than normal. Values derived from one uptake kit behaved similarly to those of the "ideal" model. The failure of the free thyroxin index to compensate for increases in thyroglobulin concentration is therefore a function of specific uptake methods rather than a characteristic of free thyroxin index per se. However, the free thyroxin index is not a true reflection of thyroid status in patients with thyroglobulin deficiency. The model suggests that a curvilinear relationship between triiodothyronine uptake and the concentration of thyroxin-binding globulin can result in a free thyroxin index that corrects for increases in thyroglobulin binding capacity.