Measurement of serum TSH and thyroid hormones in the management of treatment of thyroid carcinoma with radioiodine.

1. Serum levels of TSH, thyroxine (T4) and triiodothyronine (T3) have been measured during treatment and follow-up of patients with thyroid carcinoma. 2. Serum TSH and thyroid hormone levels were initially normal. Three weeks after total thyroidectomy, TSH was elevated in about 50% of patients and after a subsequent therapy dose of 131I it exceeded 30 mU/l in 90% of patients. Occasionally, TSH did not rise until after the second dose of 131I. Low serum T4 and T3 concentrations were associated with the increase of TSH. 3. Withdrawal of l-thyroxine replacement treatment in athyreotic patients for four weeks before test doses of 131I, led to falls of serum T4 and T3 concentration and a progressive rise of serum TSH after the first weeks but there was considerable variation in the final level reached. In the majority, values greater than 30 mU/l were attained despite some patients having received l-thyroxine for many years. A few patients on prolonged thyroxine maintenance had little or no increase in TSH despite considerable reduction in serum T4 and T3 concentrations. 4. The 131I concentration (muCi/g) developed in tumour tissue was also examined in relation to the serum TSH level. In general a tumour should not be considered as incapable of concentrating 131I adequately until serum TSH levels have exceeded 30 mU/l.