PATTERN OF SERUM THYROXINE, TRIIODOTHYRONINE AND THYROTROPHIN AFTER TREATMENT OF THYROTOXICOSIS

SUMMARY Serial measurements of the levels of thyroxine (T4), triiodothyronine (T3) and thyrotrophin (TSH) have been made in sixty-two patients with thyrotoxicosis during the management of their disease. Thirty-six patients received 131I, twenty antithyroid drugs and six underwent thyroidectomy. All have been followed for 1–2 years. Serum T4 concentrations were determined by a modification of the Murphy technique (N: 5–12 μg/100 ml). Serum T3 values were assessed by the method of Sterling et al. (N: 150–270 ng/100 ml). TSH was measured by radioimmunoassay (N: 0–8 μU/ml). The following hormonal and clinical patterns were observed. 1 Euthyroidism, with normal T4, T3 and TSH in forty-four patients. 2 Persistence of hyperthyroidism with high serum T3 (370–676 ng/100 ml), normal T4 and undetectable TSH, in four patients treated with 131I and in four managed with antithyroid drugs. 3 In four out of six patients, clinically euthyroid after 131I therapy, serum TSH became elevated (32–93 μU/ml). In this group serum T4 was decreased (1.3–3.6 μg/100 ml), but T3 was normal or somewhat elevated (170–348 ng/100 ml). Three of these patients eventually developed hypothyroidism and T3 levels fell below normal. 4 Four patients showed a transient (2–4 months) fall of serum T4 below the normal level. Serum T3 was normal and they were clinically euthyroid. These results show that during the management of thyrotoxicosis, serum T4 may be misleading. The measurement of T3 and TSH, in addition to T4, provides a better correlation between clinical status and laboratory findings.