INTRODUCTION AND AIM: New entities, such as 'subclinical' over- and undersubstitution, are easily diagnosed after thyroid surgery due to improved testing methods, and the incidence of thyroidectomy with lifelong hormone substitution is increasing. Thus, there is a need to review conventional replacement therapy after thyroid surgery. We investigated the adequacy of our thyroid hormone replacement therapy for three months after total-, subtotal-, and hemithyroidectomy using an upper reference limit of thyrotropin (TSH) of 4.6 mU/L. MATERIALS AND METHODS: Eighty-seven patients undergoing thyroidectomy for benign thyroid pathology participated. Levothyroxine (L-T4) treatment began five days after surgery. Preoperatively euthyroid patients received 150 microg L-T4 daily following total thyroidectomy, 100 microg L-T4 after subtotal thyroidectomy, and 50 microg L-T4 after hemithyroidectomy. Preoperatively hyperthyroid patients received 100 microg L-T4 following total thyroidectomy and 50 microg L-T4 following subtotal thyroidectomy. An average of six weeks after surgery, thyrotropin (TSH) was measured (reference limits 0.15-4.60 mU/L), and necessary dose adjustments were made. RESULTS: Of the patients who were preoperatively euthyroid, 45% with total thyroidectomy, 42% with subtotal thyroidectomy, and 17% with hemithyroidectomy required L-T4 dose adjustments. Of the patients who were preoperatively hyperthyroid, 60% of those with total thyroidectomy and all of those with subtotal thyroidectomy required L-T4 dose adjustments. CONCLUSIONS: To avoid over- and undersubstitution after thyroidectomy, an optimal replacement therapy dose is necessary. A small majority of our preoperatively euthyroid patients received adequate therapy. Endocrinological follow-up six weeks after surgery revealed the need for L-T4 dose adjustments, especially in preoperatively hyperthyroid patients. When the extent of resection was similar for hyperthyroid and euthyroid patients, the same initial dose of L-T4 was justified.