Thyroid hormones (TH) play a central role in regulating gene expression and complex biological processes, particularly in growth and reproduction across various environments. TH serve distinct roles in different species, including inducing metamorphosis in amphibians and teleost fishes, governing metabolic processes in mammals, and acting as effectors of seasonality. Recent evidence suggests a shared ecological role of TH across vertebrates, potentially extending to a significant portion of bilaterian species. TH ensure that ontogenetic transitions align with environmental conditions, particularly in terms of energy expenditure, helping animals to match their ontogenetic transition with available resources. This review outlines the fundamental mechanisms of thyroid signaling and explores how animals use this hormonal system in natural environments. It proposes a model linking TH signaling, environmental conditions, ontogenetic trajectory, and metabolism. TH levels are tightly regulated to ensure that metabolism and energy expenditure are consistent with the environment, allowing organisms to adapt to different environments and maintain optimal fitness. However, the role of TH alone in environmental-mediated metabolic adjustments is simplistic, and other hormonal pathways likely accompany changes in TH. TH also play a crucial role during post-embryonic development, particularly in coordinating metamorphosis, ensuring metabolic coordination, organ transformation, and body growth. The ecological function of TH is linked to local adaptation, and disruptions to TH signaling can lead to disharmonies and reduced ecological quality.Thyroid hormones (TH) play a central role in regulating gene expression and complex biological processes, particularly in growth and reproduction across various environments. TH serve distinct roles in different species, including inducing metamorphosis in amphibians and teleost fishes, governing metabolic processes in mammals, and acting as effectors of seasonality. Recent evidence suggests a shared ecological role of TH across vertebrates, potentially extending to a significant portion of bilaterian species. TH ensure that ontogenetic transitions align with environmental conditions, particularly in terms of energy expenditure, helping animals to match their ontogenetic transition with available resources. This review outlines the fundamental mechanisms of thyroid signaling and explores how animals use this hormonal system in natural environments. It proposes a model linking TH signaling, environmental conditions, ontogenetic trajectory, and metabolism. TH levels are tightly regulated to ensure that metabolism and energy expenditure are consistent with the environment, allowing organisms to adapt to different environments and maintain optimal fitness. However, the role of TH alone in environmental-mediated metabolic adjustments is simplistic, and other hormonal pathways likely accompany changes in TH. TH also play a crucial role during post-embryonic development, particularly in coordinating metamorphosis, ensuring metabolic coordination, organ transformation, and body growth. The ecological function of TH is linked to local adaptation, and disruptions to TH signaling can lead to disharmonies and reduced ecological quality.