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Results of new research suggest that in Canada, hypothyroidism in pregnancy may be overtreated.
The findings, published in CMAJ, are from a retrospective cohort study of pregnancies in Alberta, Canada.
For the study, the authors used administrative data from the Alberta Perinatal Health Program database on women aged 15 to 49 who delivered in that province between October 1, 2014 and September 30, 2017.
The reference range for thyroid stimulating hormone (TSH) in pregnancy was defined as 0.10 to 4.00 mIU/L, based on guidelines from the American Thyroid Association (ATA).
Subclinical hypothyroidism was defined as TSH 4.01 to 9.99 mIU/L and overt hypothyroidism as TSH ≥ 10.00 mIU/L. TSH measurements in the 2.51 to 4.00 mIU/L range also were analyzed.
The women had 188,490 deliveries during the study period, and in 117,252 pregnancies, thyroid testing was done at least once. In 59% of those pregnancies, TSH was measured, in 5.7%, free thyroxine was measured, and in 2.5%, thyroid peroxidase antibodies were measured.
Multiple logistic regression showed that TSH measurement was more likely in women aged 35 or older who were nulliparous, from an urban area, had gestational hypertension, or who had other medical disorders.
TSH measurement was less likely in women who smoked. Testing was most common during gestational weeks 5 and 6.
In the majority of pregnancies (92.5%), TSH levels on the first test were in the normal range for pregnancy (0.10 to 4.00 mIU/L) whereas in 4.0%, the results showed subclinical hypothyroidism and in 1.5%, the results showed overt hypothyroidism.
Thyroid hormone therapy was prescribed for women in 4.5% of pregnancies in which TSH testing was performed. They had a median of four TSH measurements during pregnancy.
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 postpartum for 44.6% of women who started therapy during pregnancy.
“The practice of TSH testing early in the first trimester,” the authors said, “may be resulting in overdiagnosis and unnecessary thyroid hormone therapy during and after pregnancy.”
In Canada, they concluded, “clinical practice guidelines are needed to give clinicians a stepwise approach, based on the best existing evidence, for deciding whether and when TSH testing should occur.”