Studies on replacement and suppressive dosages of 1-thyroxine.

Serum thyrotropin (TSH) levels were studied in 55 hypothyroid patients in order to determine adequate replacement and suppression dosages of 1-thyroxine (T4). In accordance with previous reports it was found that most patients had normal TSH levels and were clinically euthyroid at daily doses of 0.10-0.15 mg T4. None of the patients required a dose exceeding 0.20 mg. When the TSH levels normalized, serum thyroxine and serum triiodothyronine also fell to levels within their normal ranges. The effectiveness of various doses of T4 in suppressing the temporary rise in serum TSH concentration normally induced by thyrotropin-releasing hormone was examined in 57 patients treated with T4 for atoxic goitre or after subtotal surgical removal of such a goitre. The rise in TSH was not usually inhibited by a T4 dose of less than 0.20 mg, a finding which at least theoretically has implications for the adequate suppressive dose of T4.