Triiodothyronine and thyroxine interrelationships in health and disease

With the recent development of radioimmunoassay techniques for the measurement of serum triiodothyronine (T3) concentration, new concepts have arisen regarding the biologic role of T3 in health and disease and its interrelationships with thyroxine (T4). An awareness of the influence of clinical conditions that affect binding of thyroid hormone to plasma proteins is required in the interpretation of moderately increased or decreased serum T3 values. Hormone preparations containing T3 may produce transient increases in T3 concentration into the hyperthyroid range. Measurements of serum T3 concentration appear to be particularly indicated in clinical situations in which hyperthyroidism is suspected but serum T3 resin uptake and serum T4 values are normal, to exclude the T3-toxicosis syndrome. Also, when serum T4 values are in the hypothyroid range, measurement of serum T3 as well as serum thyrotropin (TSH) concentrations can lead to recognition of abnormalities in thyroid gland biosynthesis. Before a diagnosis of hypothyroidism is made on the basis of a low serum T3 value, one must exclude a variety of clinical nonthyroidal conditions that can result in changes in plasma T3 protein binding or impaired peripheral conversion of T4 to metabolically active T3 without producing a hypometabolic state.