BACKGROUND: Serum TSH is the target hormone by which adequate thyroid hormone supply can easily be monitored in patients with primary hypothyroidism. It is however controversial when TSH should be measured before thyroxine dose adjustments are made: 4 to 8 weeks are recommended. We looked at the time required to reach stable TSH levels in hypothyroid patients. METHODS: We studied patients with newly diagnosed hypothyroidism (TSH >10 mU/l and fT4 <12.3 pmol/l). Treatment was initiated with thyroxine 50 μg/d if there was a history of cardiac disease and 100 μg/d otherwise. Blood pressure, weight and TSH, fT4, fT3, cystatin C and creatinine were measured once a week. Thyroxine dose was increased by 25 μg every 8 weeks until TSH normalised. RESULTS: 12 patients with a mean TSH at baseline of 57.6 mU/l (range 11.3–151.8 mU/l) gave informed consent. They were followed for 8 to 24 weeks. After adjusting for the number of observation periods for each patient, the mean time to achieving stable TSH was 3.5 weeks (95% CI, 2.6-4.3 weeks), whereby stable TSH was defined as the value reached on a certain replacement dose after which TSH fluctuated by no more than +/- 2 mU/l during the remaining weeks of an 8 week observation period (median TSH at study end 4.7 mU/l). CONCLUSIONS: TSH did not change significantly after a mean of 3.5 weeks after the introduction of thyroxine. Dose changes can therefore be made after 4 weeks of treatment, longer periods are unnecessary.