Background:

The effect of subclinical hypothyroidism (SCH) on adverse perinatal outcomes is unclear, and thyroid-stimulating hormone (TSH) reference values in pregnancy are controversial.

Objective:

To evaluate the effects of thyroid peroxidase antibody (TPOAbs) negative SCH on perinatal outcomes according to the different TSH reference values.

Methods:

A total of 554 pregnant women, including 509 euthyroid and 45 subclinical hypothyroid (TSH > 3 mIU/L) pregnant women, were included in this prospective case-controlled study. All pregnant women were in the third trimester and were TPOAbs negative. Thyroid functions were evaluated using trimester-specific reference values recommended by the American College of Obstetrics and Gynecology (ACOG) (TSH > 3 mIU/L) and the American Thyroid Association (ATA) (TSH > 4 mIU/L) guidelines.

Results:

Neonatal mortality in subclinical hypothyroidism with a TSH upper limit of 4 mIU/L was significantly lower than in the euthyroid group (2 (0.4%) vs 1 (4.5%); p=0.009). There was no significant difference in terms of adverse maternal and perinatal outcomes in SCH and euthyroid pregnant women in both TSH reference values. There was no significant correlation between TSH values and delivery weeks of pregnant women with preterm delivery (r=0.169, p=0.146).

Conclusions:

In this study, using different baseline TSH values recommended by the 2020 ACOG and 2017 ATA guidelines in the third trimester of pregnancy for the diagnosis of subclinical hypothyroidism, it was shown that there was no significant relationship between cases of subclinical hypothyroidism with negative TPOAbs and adverse perinatal outcomes.

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