Two new studies have added to the evidence about treating pregnant women with subclinical hypothyroidism.
According to a US study published in the BMJ, treated pregnant women were significantly less likely to experience pregnancy loss than untreated women.
The observational study of more than 5,000 women between 2010 and 2014 defined subclinical hypothyroidism as thyroid-stimulating hormone (TSH) levels in the range 2.5-10 mIU/L and normal thyroid hormone levels.
Professor John Walsh, from the University of Western Australia and a consultant endocrinologist at Sir Charles Gairdner Hospital, said the important finding was in further breakdown of the study group by TSH levels.
“The benefit was only in women with pre-treatment TSH levels 4.1-10 mIU/L who had a 55% reduction in pregnancy loss. There was no benefit in women in the 2.5-4.0 mIU/L range.”
The study also found a downside to treatment – treated women were more likely to experience complications including preterm delivery, gestational diabetes and pre-eclampsia.
“My take home message is regarding the women with TSH levels in the 2.5-4.0 mIU/L range. I think we’ve been over treating otherwise well women; it’s just the reference range that has been wrong,” Professor Walsh said.
“However pregnancy loss is important and awful so if a pregnant women presents with TSH levels in the 4.1-10 range, you wouldn’t leave them untreated.”
He said additional information from thyroid peroxidase antibodies would help better define which women would benefit from treatment.
“Positive antibody tests suggest mild thyroid disease and these women may not have sufficient reserves for pregnancy and could benefit from treatment,” he said.
A second study published in the New England Journal of Medicine found no evidence of cognitive benefit in the children of women treated with levothyroxine for subclinical hypothyroidism during their pregnancy.
IQ scores, annual developmental testing scores, and the result of behavioural and attention assessments did not differ between the children of treated or untreated women.
Professor Walsh said controversy remained about whether all pregnant women should be screened for thyroid disease, but if they are screened, clinicians should focus on identifying the small number of women with overt hypothyroidism.