When should TSH levels be checked after starting thyroid hormone replacement therapy?

After initiating thyroid hormone replacement therapy, TSH levels should ideally be checked in approximately 6 weeks. This timing corresponds to the physiological response of the body to the increased levels of thyroid hormone. Thyroid hormones have a significant effect on metabolism and the regulation of the hypothalamic-pituitary-thyroid axis, but it typically takes several weeks for new hormone levels to stabilize and for the body to adjust to the therapy.

At the 6-week mark, enough time has passed for the therapy to exert an effect on TSH levels, allowing for an accurate assessment of the therapy's efficacy and the need for any potential dosage adjustments. Checking levels any earlier, such as in 2 weeks or 1 month, may not provide reliable information on how well the treatment is working, as the effects of the new medication may not be fully realized. A follow-up at 12 weeks could provide a second assessment but is not the initial follow-up period recommended for evaluating the adequacy of therapy.