Suppression of T4 secretion in a metastatic follicular carcinoma.

Despite total thyroidectomy, a patient with metastatic follicular carcinoma of the thyroid remained biologically euthyroid three months after stopping thyroxine (T4) therapy. Thyroid hormone production was investigated by means of a modified tri-iodothyronine (T3) suppression test, in which serum T4 was used as a suppression marker. After three weeks of oral T3 (Cytomel) therapy (50 micrograms/day), the serum T4 decreased from normal (108 nmol/L) to undetectable values. However, even though suppressive therapy was effective in preventing TSH dependent hormone secretion by the tumor, it did not prevent tumor growth and the eventual death of the patient.