Serum Triiodothyronine in Health and Disease

SUMMARY Using a double antibody radioimmunoassay (RIA) for the measurement of serum triiodothyronine (T3), the present study was undertaken to define the role of this hormone in normal and abnormal thyroid states. Other tests of thyroid function performed included measurement of serum thyroid stimulating hormone (TSH, euthyroid range <0.2‐3.6 μU/ml by RIA) and free thyroxine index (FTI, 4.6‐10.0). In forty‐six normal subjects (aged 18‐40 years) serum T3 ranged from 75 to 177 with a mean value of 128 ng/100 ml. This differed significantly (P<0.01) from that of the mean T3 level of nine subjects taking oral contraceptives (174 ng/100 ml). The administration of 200 μg i.v. synthetic thyrotrophin releasing hormone to six normal subjects produced a significant serum T3 elevation in all with a mean increase at 120 min of 56%. Serum T3 values in hyperthyroidism were as follows: diffuse goitre (n= 14, mean 600 ng/100 ml, range 290‐1150); multinodular goitre (n= 8, mean 283, range 179‐440); Solitary nodule (n= 4, mean 258, range 195‐305): T3‐toxicosis (n= 2, mean 262). There was no overlap with the normal range. Twenty patients with severe primary hypothyroidism (FTI 0.2‐4.3, serum TSH 83‐1546 μU/ml) had a mean serum T3 level of 42 ng/100 ml. Eighteen patients were studied following drug or radioiodine treatment of thyrotoxicosis. All were clinically euthyroid but had elevated serum TSH (6.0‐84 μU/ml). Serum T3 in this group was normal (87‐185 ng/100 ml) whereas T4 was low in eleven. It is concluded that serum T3 measurements provide a reliable index of thyroid function, facilitate the diagnosis of T3‐toxicosis and correlate with the clinical status during and after treatment of thyrotoxicosis in a more appropriate manner than do T4 measurements which may be misleading.