Subclinical Hypothyroidism May Be Overdiagnosed, Overtreated in Pregnancy

Pregnant woman having a blood test
Nurse getting blood from on a pregnant woman
Current clinical management may contribute to overdiagnosis and overtreatment of subclinical hypothyroidism during pregnancy and postpartum.

Thyrotropin (TSH) testing is performed in more than half of all pregnancies in women without a known history of prepregnancy thyroid disease and thyroid hormone therapy for subclinical hypothyroidism is often initiated after a single TSH measurement, according to study results published in CMAJ.

In light of the physiologic changes in thyroid function during pregnancy, there is much debate regarding the upper TSH reference limit and the benefit of levothyroxine treatment for pregnant women. The goal of the current study was to assess the routine clinical management of thyroid testing and treatment during pregnancy and after birth in women without prepregnancy thyroid disease.

The retrospective cohort study included data on pregnant women aged 15 to 49 years who did not have a prior history of thyroid disease and who delivered in Alberta, Canada, between October 1, 2014, and September 30, 2017.

The researchers calculated the percentage of pregnancies with thyroid testing and the frequency of each specific thyroid test. The reference range for TSH in pregnancy was defined as 0.10 to 4.00 mU/L, subclinical hypothyroidism was defined as a TSH level of 4.01 to 9.99 mU/L, and overt hypothyroidism was defined as a TSH level of ≥10.00 mU/L.

Of 188,490 women included in this study, 111,522 (59.2%) had TSH level and 10,685 (5.7%) had free thyroxine level measured at least once during the pregnancy. The most common time for thyroid testing was at gestational week 5 to 6.

In total, 5050 (4.5%) women with TSH testing were started on thyroid hormone therapy, which was initiated at a median gestational age of 7 weeks. Women who initiated thyroid hormone therapy during pregnancy had their first TSH test earlier in gestation compared with women who had a TSH test who did not receive thyroid hormone therapy (P <.001).

Of 4417 women with subclinical hypothyroid values, 2451 (55.5%) received thyroid hormone therapy. Repeat TSH testing was completed in less than half (40.1%) of women whose initial TSH was in the subclinical hypothyroid range but who were not immediately treated, and in most cases (67.9%) it showed TSH normalization. Among women with subclinical hypothyroidism, patient characteristics did not differ between those who were and were not treated.

Thyroid hormone therapy was continued during the first year after delivery for 2252 (44.6%) of the 5050 women who began therapy during their pregnancy.

The study had several limitations, including the lack of detailed patient information on the clinical indications for thyroid testing or treatment, potential for inclusion of patients with preexisting thyroid disease, and unmeasured differences between women with subclinical hypothyroidism who were and were not treated.

“The findings of our study suggest that current practice patterns may contribute to overdiagnosis of hypothyroidism and overtreatment during pregnancy and post partum,” concluded the study authors.

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Yamamoto JM, Metcalfe A, Nerenberg KA, Khurana R, Chin A, Donovan LE. Thyroid function testing and management during and after pregnancy among women without thyroid disease before pregnancy. CMAJ. 2020;192:E596-602.