Fetal Cardiac Morphology and Functional Changes in Fetuses of Well‐Controlled Gestational Subclinical Hypothyroidism Pregnancies

Background Although gestational subclinical hypothyroidism is frequently encountered, its impact on fetal cardiac morphology and function remains insufficiently investigated. The aim of our study is to evaluate the effects of well‐controlled gestational subclinical hypothyroidism on fetal cardiac geometry and myocardial function during the second and third trimesters of pregnancy. Methods This prospective cross‐sectional study included fetuses of euthyroid pregnant women and pregnant women with well‐controlled gestational subclinical hypothyroidism receiving levothyroxine therapy. A total of 73 fetuses were evaluated in the second trimester and 61 fetuses in the third trimester. Comprehensive fetal echocardiography was performed, including M‐mode, pulsed‐wave Doppler, and tissue Doppler imaging. Results Relative wall thickness (RWT) was significantly increased in the gestational subclinical hypothyroidism group during both the second and third trimesters. In the third trimester, fetuses in the study group demonstrated significantly higher mitral peak E and A velocities, increased left ventricular myocardial performance index (MPI), prolonged isovolumetric contraction (IVCT) and relaxation times (IVRT), and altered tissue Doppler diastolic indices compared to controls. Concentric remodeling was the predominant left ventricular geometric pattern in the study group. Regression analysis identified RWT, mitral E velocity, and left ventricular MPI as the main determinants of altered ventricular geometry in the third trimester. Conclusion Despite adequate levothyroxine treatment and biochemical euthyroidism, gestational subclinical hypothyroidism is associated with subtle but significant alterations in fetal cardiac geometry and myocardial function. Thus, fetal cardiac development is highly sensitive to maternal thyroid status and that functional and geometric remodeling may occur even in treated gestational subclinical hypothyroidism pregnancies. Trial Registration number: E‐11095095‐050.04‐237766