Amiodarone disrupts thyroid hormone signaling networks governing early heart development in the chick embryo

Thyroid hormones (THs) are indispensable regulators of vertebrate embryogenesis, orchestrating signaling networks that direct cardiac morphogenesis. To investigate how disruption of this pathway impacts early development, fertilized Gallus domesticus eggs were exposed to amiodarone, a potent TH receptor (THR) antagonist and deiodinase inhibitor. Treated embryos displayed pronounced defects, including impaired heart looping, edema, increased apoptotic regions and sustained bradycardia during Hamburger-Hamilton stages 12 to 18. By day 10, histology revealed significant thinning of ventricular and atrial walls, with the compact ventricular layer reduced by ∼22 % while trabeculae were relatively preserved. These changes were accompanied by reduced acetylcholinesterase activity, indicating impaired neurocardiac regulation. Molecular analyses showed broad downregulation of WNT11, GATA4/5, TBX20, HAND2, BMP4, SHH, FGF8, MYOD, and MYOSIN, together with decreased PCNA and compensatory upregulation of GATA6. Interestingly, discrepancies between transcript and protein levels suggested post-transcriptional control under hypothyroid conditions. In silico promoter scanning identified thyroid hormone response elements within HAND2, GATA6, TBX5, PITX2, and BMP4, linking maternal TH signaling directly to lateral plate mesoderm gene networks. Flow cytometry and whole-mount immunolocalization confirmed reduced expression and altered localization of MYOD and MYOSIN, including loss of the normal heart-tube-restricted MYOD signal. Collectively, these findings establish that TH signaling networks coordinate structural, functional, and molecular programs essential for early cardiogenesis. Amiodarone-induced THR blockade recapitulates developmental hypothyroidism, providing mechanistic insight into how maternal TH deficiency or endocrine-disrupting exposures may contribute to congenital heart defects.