Embryonic Thyroid Hormone Insufficiency Causes Structural Anomalies in the Embryo of Domestic Chick, <i>Gallus domesticus</i>

Thyroid hormone (TH) is essential for growth and development, yet its specific role during embryogenesis remains incompletely understood. This study investigates the impact of TH deficiency, induced by thiourea, a known inhibitor of thyroid peroxidase (TPO), on the development of domestic chicks. Thiourea was administered before thyroid gland formation, and its presence in treated embryos was confirmed through liquid chromatography-mass spectrometry. In silico docking revealed a strong interaction between thiourea and the CCP-like domain of TPO, which was corroborated by TPO activity assays showing reduced enzyme function. This reduction in enzyme activity led to lower embryonic TH levels and increased thyroid-stimulating hormone (TSH) secretion. Morphological analysis of newly hatched chicks revealed significant structural anomalies, particularly in lateral plate mesoderm-derived structures, including omphalocele, limb deformities, anophthalmia and craniofacial defects. Alcian blue and Alizarin red staining demonstrated reduced ossification in ribs and forelimbs, while histological analysis showed incomplete abdominal wall closure and abnormal vertebral column development. Haematological profiling of TH-deficient newly hatched chicks revealed significantly lower blood cell counts, highlighting TH's critical role in haematopoiesis. These findings emphasise the multifaceted role of TH in embryonic development, with potential implications for understanding congenital hypothyroidism and its developmental impacts, especially in regions with limited healthcare access.