Endocrine Abstracts

Aim: Serum TSH is the target value by which adequate thyroid hormone supply can easily be monitored in patients with primary hypothyroidism. It is controversial when TSH should be measured before thyroxine dose adjustments are made, 4–8 weeks are recommended. To define the time when dose adjustments of thyroxine are feasible more clearly, we looked at the time required to reach stable TSH levels.

Methods: We prospectively studied a case series of patients with newly diagnosed hypothyroidism (TSH >10 mU/l and fT₄ <12.3 pmol/l). Treatment was initiated with standardised doses of thyroxine, 0.05 mg/day if there was a history of cardiac disease and 0.1 mg/day in all other cases. Blood pressure, pulse, weight, TSH, fT₄, fT₃, cystatin C and creatinine were recorded once weekly on the same day for a total of 8 weeks. Thyroxine dose was kept constant until the change in TSH between one week and the next was <2 mU/l, and it was increased by 0.025 mg every 8 weeks until TSH normalised.

Results: Twelve (six male) patients, 44.6+11.6 (mean+S.D.) years old with a median TSH at baseline of 57.6 mU/l (range 11.2–151.8 mU/l) and a median cystatin C at baseline of 0.77 mg/l (range 0.47–1.3 mg/l) gave informed consent. The patients were observed for a minimum of 8 weeks and a maximum of 24 weeks (eight patients for 8 weeks, one patient for 16 weeks and two patients for 24 weeks), resulting in 19 observation periods. Nine patients were euthyroid by the end of the study, the remaining three patients could not participate for a further 8 weeks due to work responsibilities. After adjusting for the number of observation periods for each patient, the mean time to achieving stable TSH (defined as a change in TSH of <2 mU/l per week) was 3.45 weeks (95% CI, 2.44–4.46 weeks).

Conclusion: TSH does not seem to change significantly after a mean of 3.5 weeks after an alteration of thyroxine dose. Dose adaptations can be made after 4 weeks of treatment without having to wait 6–8 weeks.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.