The Effects of Maternal Subclinical Hypothyroidism on Fetal Thymus Size: A Prospective Study

Objective: This study investigated the impact of maternal subclinical hypothyroidism on fetal thymus size and development and explored how inadequate thyroid hormone production in pregnant women affects the fetal thymus. Methods: Conducted at the Giresun Obstetrics, Gynecology, and Pediatrics Training and Research Hospital, this case-control study involved 86 pregnant women, 43 with hypothyroidism and 43 without. Maternal thyroid function was assessed using TSH and free T4 levels, and fetal thymus size and thymus-thorax ratio were measured using ultrasound. Exclusion criteria were chronic hypertension, gestational hypertension or eclampsia, multiple pregnancies, infectious diseases, renovascular diseases, diagnosed with hypothyroidism prior to pregnancy and other endocrine disorders, fetal cardiac diseases, and morbid obesity. Data collected included maternal age, gestational week, number of pregnancies, parity, number of living children, thyroid-stimulating hormone (TSH) and Free thyroxine 4 (T4) levels, and fetal thymus measurements (transverse diameter and thymus/thorax ratio). Statistical analyses were performed using the Mann-Whitney U test and logistic regression analysis. The relationships between TSH, thymus diameters, thorax diameters, and the thymus-thorax ratio were evaluated using Spearman's correlation coefficient. Results: The thymus-thorax ratio was significantly reduced in the hypothyroid group (p = 0.003). Logistic regression analysis identified TSH as an independent risk factor for a low thymus-thorax ratio, with each unit increase in TSH associated with a 1.345-fold higher likelihood of having a low thymus-thorax ratio. A significant negative correlation was found between TSH levels and the TTR ratio (Spearman's correlation coefficient r = -0.338, p = 0.001). Conclusions: An association was identified between maternal TSH levels and the thymus-thorax ratio, with increasing TSH levels correlating with a decrease in the thymus-thorax ratio. Regular monitoring of thyroid hormone levels during pregnancy and appropriate replacement treatment in cases of deficiency are crucial for optimal fetal thymus development. Further multicenter studies are needed to confirm these findings and investigate the long-term implications of altered fetal thymus development.