Gestational subclinical hypothyroidism is overdiagnosed and overtreated

TSH testing early in the first trimester may be resulting in overdiagnosis and unnecessary thyroid hormone therapy during and after pregnancy, according to Canadian research.

Most women who had TSH levels suggesting subclinical hypothyroidism in an initial test, subsequently reverted to normal levels according to a study of 188,000 pregnancies.

The retrospective cohort study of pregnancies in Alberta between 2014 and 2017 found that 59% of  women had at least one TSH measurement, the most common time for testing being at gestational week five to six.

The first TSH measurement was in the normal pregnancy range for 93% of women, whereas  4417 (4.0%) of women had a first TSH measurement in the “subclinical hypothyroid range,” ( 4.01 and 9.99 mIU/L), and 1709 (1.5%) had a first TSH measurement in the overt hypothyroid pregnancy range.

In around 5% of pregnancies with TSH testing, women were started on thyroid hormone therapy. This included 2451 (55.5%) of the 4417 women with subclinical hypothyroid values.

Among women with first TSH measurements in the subclinical hypothyroid range who were not immediately treated, the repeat TSH measurement was below 4.00 mIU/L or below in 68% of cases.

For the women who started thyroid hormone treatment, almost half (44.6%) continued with the treatment after giving birth, and almost one-third (31.5%) received two or more prescriptions in the first postpartum year.

Study lead author Dr Lois Donovan, an endocrinologist at the Cumming School of Medicine, University of Calgary said the findings showed why screening for subclinical hypothyroidism in pregnancy is inappropriate.

She noted that treatment of minor TSH elevation (i.e., subclinical hypothyroidism) or thyroid peroxidase antibodies in pregnancy has no benefit for the mother, the neonate or the developing child.

“This raises concerns about overmedicalisation during pregnancy, given that minor, untreated TSH elevation usually normalised, as indicated by repeat measurement,” Dr Donovan and colleagues wrote in the CMAJ.

“The frequent postpartum continuation of thyroid hormone therapy for those who started it during pregnancy adds to this concern.”

The authors said 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.

In Australia, current guidance from the Royal Australian and NZ College of Obstetrics and Gynaecology (RANZCOG) is that targeted thyroid function testing with serum TSH should be performed in early pregnancy for women with symptoms of thyroid disease or a personal history of thyroid disease.

“There is insufficient evidence to support universal TSH screening and treatment of subclinical hypothyroidism in pregnancy,” it states.

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