Thyroxine kinetics in nonthyroidal illnesses.

Free T4 estimates by equilibrium dialysis appear to reflect serum free T4 levels in vivo in patients with nonthyroidal illnesses. However, patients with either elevated or modestly reduced free T4 values appear to be clinically euthyroid and have relatively normal free T4 availability to tissues as judged by the normal production rates of reverse T3 from T4, and by the minimally altered TSH status. Thus, increased or modestly reduced serum free T4 levels by equilibrium dialysis in patients with nonthyroidal illnesses do not appear to indicate the presence of overt T4 excess or deficiency, respectively, and, therefore, the clinician should strive to avoid unnecessary and potentially harmful therapy in these patients. Conversely, concurrent thyroid gland disease may be present which must be accurately diagnosed and treated to alleviate the morbidity induced by hyperthyroidism or hypothyroidism in sick patients.