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Obstet Gynecol. 2007 May;109(5):1129-35.

Perinatal significance of isolated maternal hypothyroxinemia identified in the first half of pregnancy.

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1
Department of Obstetrics and Gynecology, the University of Texas Southwestern Medical Center, Dallas, Texas 75390-9032, USA. brian.casey@utsouthwestern.edu

Abstract

OBJECTIVE:

To establish pregnancy-specific free thyroxine thresholds and to assess perinatal effects associated with isolated maternal hypothyroxinemia identified in the first half of pregnancy.

METHODS:

Stored serum samples from 17,298 women who previously underwent thyroid-stimulating hormone (TSH) screening in the first half of pregnancy were analyzed for free thyroxine (T(4)) concentrations and thyroid peroxidase antibodies. Women with a free T(4) below 0.86 ng/dL but a normal-range TSH were identified to have isolated maternal hypothyroxinemia. Pregnancy outcomes in these women were compared to those with a normal TSH and free T(4). Thyroid peroxidase antibody status and the relationship between TSH and free T(4) were analyzed for these women and women with subclinical hypothyroidism.

RESULTS:

Isolated maternal hypothyroxinemia was identified in 233 women (1.3%). There were not any excessive adverse pregnancy outcomes in these women. Positive thyroid peroxidase antibody assays (greater than 50 international units/mL) were similar in normal women (4%) and those with isolated hypothyroxinemia (5%) but were greater in women with subclinical hypothyroidism (31%, P<.001). There was a negative correlation between TSH and free T(4) in normal women (r(s)=-0.19, P<.001) and those with subclinical hypothyroidism (r(s)=-0.11, P=.007). The correlation in women with isolated hypothyroxinemia was not significant.

CONCLUSION:

Isolated maternal hypothyroxinemia has no adverse effects on perinatal outcome. Moreover, unlike subclinical hypothyroidism, there was a low prevalence of thyroid peroxidase antibodies and no correlation between TSH and free T(4) levels in women with hypothyroxinemia, leading us to question its biological significance.

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