Subclinical Hypothyroidism in Pregnancy Usually Transient

Marlene Busko

December 20, 2013

Three-quarters of women with subclinical hypothyroidism during pregnancy had normal thyroid function 5 years later, in a new study published in the December issue of the Journal of Clinical Endocrinology & Metabolism.

"Increasing numbers of pregnant women with milder forms of thyroid dysfunction are being treated with L-thyroxine, and we have demonstrated that many of these women are likely to be able to stop L-thyroxine after pregnancy," write the researchers, led by Beverley M. Shields, University of Exeter, UK.

"Our findings suggest that if L-thyroxine is prescribed for subclinical hypothyroidism, thyroid function should be reassessed after pregnancy to confirm whether continuation of L-thyroxine is necessary."

They suggest that women with mild thyroid dysfunction should be reevaluated about 6 weeks after delivery, based on current guidelines that recommend this follow-up time for women with a diagnosis of actual clinical hypothyroidism before pregnancy. "This would seem a reasonable point for reassessment in women with subclinical hypothyroidism diagnosed and treated during pregnancy," they write.

Based on trimester-specific reference ranges for thyroid hormone levels, as many as 15% of pregnant women are being diagnosed with subclinical hypothyroidism, Shields and colleagues explain. Mild thyroid dysfunction has been linked to impaired neuropsychological development in offspring and adverse obstetric outcomes — including miscarriage, premature birth, gestational hypertension, and neonatal death — and so guidelines recommend treatment with L-thyroxine.

Specifically, the recent Endocrine Society guidelines recommend that all pregnant women with subclinical hypothyroidism be treated with L-thyroxine. The American Thyroid Association guidelines recommend that pregnant women with subclinical hypothyroidism and detectable thyroid-peroxidase antibodies receive L-thyroxine.

However, there are no data to indicate whether treatment should be continued long term, and long-term treatment is not addressed in current guidelines.

The researchers hypothesized that most cases of subclinical hypothyroidism and maternal hypothyroxinemia resolve after pregnancy, because the physiological changes in pregnancy that predispose women to thyroid deficiency are likely to be temporary.

They analyzed data from 523 healthy pregnant women with no known thyroid disorder recruited into the Exeter Family Study of Childhood Health between 1999 and 2004.

Levels of thyroid-stimulating hormone (TSH), free T4, free T3, and thyroid peroxidase antibodies were determined at 28 weeks of pregnancy and, on average, 4.9 years after delivery.

During pregnancy, 65 of the 523 women (12.4%) had subclinical hypothyroidism (TSH > 3 mIU/L). At the 5-year follow-up, 49 of the 65 women (75.4%) who had subclinical hypothyroidism during pregnancy had normal thyroid function.

Only 2 of 44 women (4.4%) with hypothyroxinemia during pregnancy had high TSH levels (> 4.5 mIU/L) after pregnancy.

Among women with subclinical hypothyroidism during pregnancy, those who also had thyroid-peroxidase antibodies during pregnancy were more likely to have persistently elevated TSH after pregnancy.

Because there is a high risk of recurrence of subclinical hypothyroidism during a subsequent pregnancy, women should be tested for thyroid function early on during future pregnancies, the researchers advise.

J Clin Endocrinol Metab 2013;98:E1941-E1945. Abstract

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