BACKGROUND: Radioiodine treatment is effective for the removal of remnant thyroid tissues after thyroidectomy in patients with differentiated thyroid carcinoma. To induce the elevation of serum TSH level which facilitates the uptake of radioiodine into remnants, a 4 to 6 week interval between thyroidectomy and radioiodine administration has been established. During the period of preparation, most patients have experienced overt symptoms of hypothyroidism which have led to the development of alternative strategies. Some reports have suggested that the interval could be reduced to about 3 weeks with less symptoms. We reevaluated the adequate time needed for the elevation of serum TSH level above 30microU/mL after thyroidectomy. METHODS: Forty five patients who had undergone total thyroidectomy for differentiated thyroid carcinoma were investigated. Serum TSH and free T4 levels were measured one or more times within 3 weeks after operation(total 97 blood samples). Eighty nine blood samples were obtained within 15 days. RESULTS: In 41 patients (91.1%) serum TSH levels increased to 30 microU/mL until 15 days after operation. Until postoperative 21 days, serum TSH levels in all the other patients reached 30microU/mL. In linear equation, the daily increment of serum TSH levels was 2.62microU/mL for the first 8 days after operation and 5.34micorU/mL for the next 7 days. The half-life of serum free T4 levels showed marked individual variations. CONCLUSION: Measurement of serum TSH level at about 15 days after total thyroidectomy for differentiated thyroid carcinoma may be useful in determining the time of radioiodine administration.