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Objective It is unknown if foetal gender influences maternal thyroid function during pregnancy. We therefore investigated the prevalence of thyroid disorders and determined first-trimester TSH reference ranges according to gender. Methods A cross-sectional study involving 1663 women with an ongoing pregnancy was conducted. Twin and assisted pregnancies and l-thyroxine or antithyroid treatment before pregnancy were exclusion criteria. Serum TSH, free T4 (FT4) and thyroid peroxidase antibodies (TPOAb) were measured at median (interquartile range; IQR) 13 (11–17) weeks of gestation. Subclinical hypothyroidism (SCH) was present when serum TSH levels were >3.74 mIU/L with normal FT4 levels (10.29–18.02 pmol/L), and thyroid autoimmunity (TAI) was present when TPOAb were ≥60 kIU/L. Results Eight hundred and forty-seven women were pregnant with a female foetus (FF) and 816 with a male foetus (MF). In women without TAI and during the gestational age period between 9 and 13 weeks (with presumed high-serum hCG levels), median (IQR range) serum TSH in the FF group was lower than that in the MF group: 1.13 (0.72–1.74) vs 1.24 (0.71–1.98) mIU/L; P = 0.021. First-trimester gender-specific TSH reference range was 0.03–3.53 mIU/L in the FF group and 0.03–3.89 mIU/L in the MF group. The prevalence of SCH and TAI was comparable between the FF and MF group: 4.4% vs 5.4%; P = 0.345 and 4.9% vs 7.5%; P = 0.079, respectively. Conclusions Women pregnant with an MF have slightly but significantly higher TSH levels and a higher upper limit of the first-trimester TSH reference range, compared with pregnancies with a FF. We hypothesise that this difference may be related to higher hCG levels in women pregnant with a FF, although we were unable to measure hCG in this study. Further studies are required to investigate if this difference has any clinical relevance.

Objective: Pregnant women with autoimmune (subclinical) hypothyroidism have an increased risk of developing gestational diabetes mellitus (GDM). However, this association remains controversial in euthyroid women with thyroid autoimmunity (TAI). Therefore, the aim of the study was to determine the association between TAI and GDM in euthyroid women in a logistic regression analysis with adjustments for baseline / demographic parameters. Methods: Cross-sectional study in 1447 euthyroid women who performed their biological work-up and oral glucose tolerance test (OGTT) in our center. At median 13 (11-17) weeks of gestation, TSH, free T4 and thyroid peroxidase antibodies (TPOAb) were measured, baseline characteristics recorded and an OGTT was performed between 24-28 weeks of pregnancy. Exclusion criteria were pre-pregnancy diabetes, assisted pregnancies, and women with (treated) thyroid dysfunction before or after screening. The diagnosis of GDM was based on 2013 WHO criteria, and TAI was defined as TPOAb levels ≥60 kIU/L. Results: Two hundred eighty women were diagnosed with GDM (19.4%), 26.1% in women with TAI and 18.9% in women without TAI (p=0.096). In the logistic regression analysis, TAI was associated with GDM in women older than 30 years (adjusted odds ratio (aOR) 1.68 (95% CI, 1.01-2.78); p=0.048). Maternal age >30 years, pre-pregnancy BMI ≥30 kg/m2 and another than Caucasian background were also associated with GDM; aOR 1.93 (95% CI, 1.46-2.56); p<0.001, 2.03 (95% CI, 1.46-2.81); p<0.001 and 1.46 (95% CI, 1.03-2.06); p=0.034, respectively. Conclusions: In older pregnant women, the presence of TAI in euthyroid women was associated with GDM. In line with literature data, (higher) age and BMI were strongly associated strongly with GDM.