Thyroid hormones are central regulators of metabolic homeostasis and developmental programming. The active hormone triiodothyronine (T3) modulates transcription through nuclear receptors that recruit epigenetic cofactors to remodel chromatin and regulate metabolic gene networks. Although thyroid hormone signaling is known to influence lipid metabolism, whether it coordinates lipid droplet turnover with autophagy-related pathways during early embryonic development remains largely unknown. Here, transcriptomic profiling revealed distinct metabolic signatures between in vivo and in vitro embryos, with marked differences in fatty acid metabolism. Supplementation with 50 nM T3 enhanced blastocyst formation, particularly when applied from the 4-cell to blastocyst stages, coinciding with elevated thyroid hormone receptor expression. T3 induced robust lipid droplet remodeling, characterized by reduced droplet size, together with increased lipid-mitochondria colocalization and activation of lysosomal and mitochondrial pathways, consistent with enhanced lipid catabolism and organelle coupling. Mechanistically, inhibition of the histone acetyltransferase KAT2B/PCAF abolished T3-mediated developmental gains, reduced H3K9ac and H3K27ac, and resulted in nonselective autophagic stress rather than lipophagy. By contrast, T3 required KAT2B to stimulate cytosolic lipolysis, channel fatty acids into mitochondria, and enhance mitochondrial membrane potential. T3 also upregulated prostaglandin biosynthesis genes and improved outgrowth performance. These findings identify a thyroid hormone-KAT2B epigenetic axis that coordinates lipid droplet remodeling through lipolytic and lipophagic pathways, linking endocrine signaling to organelle crosstalk and mitochondrial activation during early embryogenesis.
