Lower Limit of TSH for Replacement T4 Treatment in Pregancy

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Question

In our daily routine practice forpregnant hypothyroid women, the T4 dosage istitrated tomaintain serum TSH concentrations of less than 2.5 mIU/ml in the first trimester (or 3 mIU/ml in the second and third trimesters) as suggested by Endo Society. However, I wonder what isthe lower limit of TSH in a pregnant women on T4 therapy? Which TSH levelmightbe tolerated or no dosage modification is required? (assuming that patient is clinically euthyroid).

Alptekin Gursoy, MD
Ankara Guven Hospital
Ankara, Turkey

Response

Probably 0.1uU/ml is a fair lower limit in women on T4 in the second and third trimester, although some normal women have lower levels in the second trimester. Also, evidence suggests that subclinical hyperthyroidism does not appear to cause problems for mother or infant during pregnancy.

L De Groot, MD