Is subclinical hypothyroidism a risk factor for gestational diabetes mellitus?

Objective: Gestational diabetes mellitus is characterized by increased blood sugar that first appears during pregnancy. Multiple articles have described a relationship between hypothyroidism/subclinical hypothyroidism (SCH) and a rise in the risk of concomitant pregnancy complications, including gestational diabetes mellitus (GDM), but the effect of SCH on pregnancy is uncertain in the literature. We clarified the contribution of SCH to GDM development. Patients and Methods: We conducted a retrospective study. From the patient records, the first 250 pregnant women who applied to our hospital for screening at 20-24 weeks and had glucose tolerance tests were included in our study. Retrospectively, all these pregnant women's first-trimester thyroid-stimulating hormone (TSH) levels were recorded. We created two groups according to the oral glucose tolerance test (OGTT) results: a case group diagnosed with GDM and a control group with average blood glucose. Their first-trimester TSH levels were compared between the two groups and defined whether they had euthyroid, subclinical hypothyroidism (TSH=2.5-5.5mIU/L) or overt hypothyroidism (TSH >5.5). Results: We diagnosed 37 of 191 patients (19.4%) with GDM. When we checked the case and control groups, the mean TSH of the GDM group was 1.8 mIU/L, and the control group was 1.7 mIU/L, but the difference was not statistically significant (p=0.121). 24.32% (n=9) of 37 pregnant women with GDM were diagnosed with subclinical hypothyroidism/hypothyroidism; this rate was as low as 14.93% (n=28) in the non-GDM group, but no statistical difference was found (p=0.21). Conclusion: It can be predicted that other accompanying factors may be the primary determinant in the development of GDM rather than subclinical hypothyroidism. Risk scales that include the first trimester TSH level should be established for the development of GDM.