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TSH lower, free T4 higher during pregnancy in women with thyroid nodules

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February 4, 2019

Thyroid nodules may be associated with lower thyroid-stimulating hormone values and higher free thyroxine levels in iodine-sufficient pregnant women, according to findings published in Thyroid.

“During the first half of pregnancy, maternal thyroid hormones play a key role in the development of the placenta and the fetus. Thyroid dysfunction occurs frequently in pregnancy and is associated with maternal or fetal complications, including miscarriage, preterm birth, preeclampsia and reduction of the child’s IQ,” M. Dolores Ollero, MD, of the department of endocrinology and nutrition at Complejo Hospitalario de Navarra in Spain, and colleagues wrote. “The assessment of thyroid function requires specific reference intervals stratified by trimesters during pregnancy, since pregnancy is associated with physiological changes in thyroid hormone production and metabolism.”

Ollero and colleagues conducted a prospective, longitudinal study with 400 iodine-sufficient pregnant women (mean age, 33.4 years) recruited from two women’s care centers in Navarra, Spain, from May 2014 to May 2016. Measurements for TSH and free T4 were collected at the first prenatal visit and then in the first trimester (at 9 weeks), second trimester (at 15 weeks) and third trimester (at 36 weeks). Thyroid echography was used to establish thyroid volumes and the presence of thyroid nodules. A significant thyroid nodule was defined as a single nodule of 1 cm or more or at least four nodules of any size.

After analysis, the researchers determined reference intervals for healthy thyroid function for TSH and free T4 in each trimester. The first trimester’s TSH reference interval was 0.13 mIU/L to 4.16 mIU/L, and the free T4 interval was 0.85 ng/dL to 1.24 ng/dL. The second trimester had a TSH reference interval of 0.31 mIU/L to 3.73 mIU/L and a free T4 interval of 0.82 ng/dL to 1.2 ng/dL. Finally, the third trimester had reference intervals of 0.58 mIU/L to 4.36 mIU/L and 0.67 ng/dL to 1.06 ng/dL for TSH and T4, respectively.

A total of 115 women in the cohort had thyroid nodules, equating to a prevalence of 28.8% (95% CI, 24.4-33.5). Women with thyroid nodules had lower TSH levels in their first (1.14 mIU/L; 95% CI, 0.53-1.75), second (1.22 mIU/L; 95% CI, 0.66-1.77) and third (1.74 mIU/L; 95% CI, 1.08-2.36) trimesters compared with the first (1.48 mIU/L; 95% CI, 0.94-2.19), second (1.45 mIU/L; 95% CI, 1.04-2.05) and third (1.93 mIU/L; 95% CI, 1.37-2.58) trimesters of women without nodules. Women with thyroid nodules also had higher free T4 levels, but only in the first trimester (1.08 ng/dL) compared with women without nodules (1.03 ng/dL; P < .001).

Of the 115 women with thyroid nodules, six had clinically significant ones. TSH values were lower for women with clinically significant nodules in the first (0.74 mIU/L; 95% CI, 0.16-1.57), second (0.64 mIU/L; 95% CI, 0.13-1.3) and third (0.92 mIU/L; 95% CI, 0.19-1.74) trimesters compared with women without nodules in their first (1.48 mIU/L; 95% CI, 0.94-2.19), second (1.45 mIU/L; 95% CI, 1.04-2.05) and third (1.93 mIU/L; 95% CI, 1.37-2.58) trimesters.0

“The current study highlights that small thyroid nodules, which are usually considered non-significant, may influence TSH levels during pregnancy,” the researchers wrote. “In our

population, pregnant women with such nodules also showed TSH levels that were significantly lower than those of pregnant women in whom an echographic study had discarded the presence of nodules. However, it has to be noted that most women with non-significant nodules had TSH values within the normal reference range.” – by Phil Neuffer

Disclosures: The authors report no relevant financial disclosures.

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