Screening in pregnancy for subclinical hypothyroidism, often defined as thyroid-stimulating hormone (TSH) greater than 2.5 mIU/L or greater than 4.0 mIU/L, is controversial. We determined the frequency and distribution of TSH testing by gestational age, as well as TSH values associated with treatment during pregnancy and the frequency of postpartum continuation of thyroid hormone therapy.
We performed a retrospective cohort study of pregnancies in Alberta, Canada. We included women without thyroid disease who delivered between October 2014 and September 2017. We used delivery records, physician billings, and pharmacy and laboratory administrative data. Our key outcomes were characteristics of TSH testing and the initiation and continuation of thyroid hormone therapy. We calculated the proportion of pregnancies with thyroid testing and the frequency of each specific thyroid test.
Of the 188 490 pregnancies included, 111 522 (59.2%) had at least 1 TSH measurement. The most common time for testing was at gestational week 5 to 6. Thyroid hormone therapy was initiated at a median gestational age of 7 (interquartile range 5-12) weeks. Among women with first TSH measurements of 4.01 to 9.99 mIU/L who were not immediately treated, the repeat TSH measurement was 4.00 mIU/L or below in 67.9% of pregnancies. Thyroid hormone was continued post partum for 44.6% of the women who started therapy during their pregnancy.
The findings of our study suggest that current practice patterns may contribute to overdiagnosis of hypothyroidism and overtreatment during pregnancy and post partum.

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