Prevalence of thyroid dysfunction in pregnant women with or without gestational diabetes

Background and aims: Data so far available on the relationship between gestational diabetes (GDM), metabolic phenotype and thyroid function during pregnancy are conflicting and deserve further investigations. Aim of this project was to assess thyroid function in a cohort of pregnant GDM or normal glucose tolerant Italian women. Materials and methods: The study was conducted at the outpatient clinic of Endocrinology and Metabolism of Pescara health service (Italy), in accordance with guidelines of the Helsinki Declaration on human experimentation. One-hundred-fifty-eight (158) pregnant caucasian women without history of thyroid disease were enrolled (105 GDM and 53 controls). GDM was investigated by OGTT 75 gr between 22 and 32 gestational weeks. Blood samples for TSH, FT3, FT4, TgAb, TPOAb and a spot urine sample for iodine measurement were collected. Blood pressure was measured, BMI was calculated and stratified into five categories (<25, 25-30, 30-35, 35-40, >40 kg/ m2); moreover, thyroid ultrasound was performed to evaluate echo pattern and thyroid volume. Statistical analyses were carried out with Stata software using non-parametric statistical tests: chi-square test, Kruskal-Wallis test, Mann-Whitney U test, Spearman’s correlation. Differences were considered statistically significant at p<0.05. Results: No difference between GDM vs control group in thyroid dysfunction (TSH, FT3, FT4) and autoimmunity was found. FT3/FT4 ratio significantly increased across BMI categories (p<0.05). Stratifying women by GDM diagnosis, this result persisted only in GDM (p=0.015 Vs p=0.22). According to that, a significant positive correlation was found between maternal weight and FT3/FT4 ratio in the study population (r=0.26, p=0.0007). This data was confirmed only in GDM (r=0.3, p=0.001). In our population, the iodine status seemed to be inadequate as indicated by urinary iodine concentration <150 μg/L [GDM 94.1 μg/L, interquartile range (IR) 112 vs control group 64.2 IR 72]. In particular, urinary iodine excretion was significantly lower in control group, especially in women with BMI >25 kg/m2 (p=0.015). Wherease, women with GDM showed higher urinary iodine concentration values regardless of BMI, suggesting better iodine intake. Conclusion: These results indicate that GDM is not associated with thyroid dysfunction or autoimmunity in our popolation. The higher FT3/ FT4 ratio observed in both obesity and GDM would be driven by the increase in peripheral conversion of FT4 to FT3. In addition, the higher iodine intake inferred from urinary iodine concentration in patients with GDM, could be explained by greater adherence to nutritional therapy and prescribed supplementation probably due to more frequent monitoring.