Interpretation of Serum Tri-Iodothyronine Levels Measured by the Sterling Technic

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Abstract
The mean serum tri-iodothyronine (T3) level was 243 ± 40 (S.D.) ng per 100 ml in 26 normal subjects, 187 ± 92 in 12 hypothyroid patients, and 671 ± 253 in 16 patients with Graves's disease. In hypothyroid patients rendered euthyroid by oral T3 treatment, serum T3 was as much as three times normal when serum thyroid-stimulating hormone was just suppressed. In hypothyroid subjects given thyroxine (T4), serum T3 was normal. Seven patients with proved hyperthyroidism and normal serum total T4 all had marked increases of serum T3. The inappropriately normal T4 value was not explained by deficiency of T4-binding globulin. Availability of serum T3 determinations adds validity to the diagnosis of hyperthyroidism in such patients, but an elevated serum T3 value should not be considered proof of hyperthyroidism without supporting evidence.