It is now recommended to use population-based trimester-specific thyroid-stimulating hormone (TSH) reference ranges, but what should be the optimal TSH level for a woman before pregnancy?
Leonard Wartofsky, MD: I think when one thinks about TSH ranges in any patient, one needs to think about what the true reference range is. Based on data from the National Health and Nutrition Examination Survey (NHANES) population surveys of tens of thousands of ostensibly normal euthyroid individuals in the United States, that range of TSH is perhaps from 0.3 to 3 mIU/L. Criteria from the US National Academy of Clinical Biochemistry lowers that upper limit to 2.5 mIU/L.
If we look at subjects who are totally free of thyroid disease, who have a negative family history of thyroid disease, who are taking no medications that affect the thyroid gland, etc, who are otherwise totally clean of any aberrancy of their thyroid function, in a Caucasian woman who is prepregnant of, let’s say, age 25, in the United States the average TSH level would be 1.39 mIU/L. In an African American woman it is approximately 1.19 mIU/L, there being less Hashimoto disease in African Americans than whites, perhaps explaining the lower TSH.
If that is the mean TSH of young women and the normal bell-shaped curve only goes up to 2.5 mIU/L, then one would want to see a TSH level in a prepregnant woman in the range of between 1 and 2 mIU/L.