Thyroid hormones (TH) are central regulators that orchestrate gene expression and complex biological processes essential for growth and reproduction in variable environments. They play distinct roles in different species, such as inducing metamorphosis in amphibians and teleost fishes, regulating metabolism 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 bilateral species. TH ensure that ontogenetic transitions align with environmental conditions, particularly in terms of energy expenditure, helping animals match their ontogenetic transition with available resources. This alignment spans post-embryonic developmental transitions common to all vertebrates and more subtle adjustments during seasonal changes. The underlying logic of TH function is to synchronize transitions with the environment. This review outlines the fundamental mechanisms of thyroid signalling and shows various ways in which animals use this hormonal system in natural environments. It proposes a model linking TH signalling, environmental conditions, ontogenetic trajectory, and metabolism. TH levels are tightly regulated in adult mammals, as excessively low or high TH levels can have detrimental effects. A negative feedback system allows TH to regulate their own levels through the hypothalamo-pituitary-thyroid (HPT) axis. TH also act as a major effector of seasonality, the process by which species adapt their physiology and reproduction to annual changes in temperature and photoperiod. TH levels are influenced by environmental factors such as resource availability, energy expenditure, thermal adaptation, and salinity acclimation. TH play a critical role in thermogenesis, helping to maintain body temperature and energy expenditure. They are also involved in thermal adaptation, with studies showing that TH levels vary in response to extreme thermic conditions. TH are also involved in salinity acclimation, with studies showing that TH levels are affected by changing salinities in several fish species. TH are involved in seasonal adaptation, with studies showing that TH levels vary in response to seasonal changes. TH are also involved in metamorphosis, a process that is known to be controlled by TH in chordates. TH are key regulators allowing organisms to acclimate to a range of environmental conditions—temperature, salinity, seasons. This role in tailoring physiology to environmental conditions is crucial for metamorphosis. TH are also involved in the regulation of seasonality, the recurring patterns of biological changes that unfold cyclically throughout the year. TH play a critical role in the regulation of seasonality, the recurring patterns of biological changes that unfold cyclically throughout the year. TH are also involved in the regulation of metamorphosis, a process that is known to be controlled by TH in chordates. TH are key regulators allowing organisms to acclimate to a range of environmental conditions—temperature, salinity, seasons. This role in tailoring physiology to environmental conditions is crucial for metamorphosis.Thyroid hormones (TH) are central regulators that orchestrate gene expression and complex biological processes essential for growth and reproduction in variable environments. They play distinct roles in different species, such as inducing metamorphosis in amphibians and teleost fishes, regulating metabolism 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 bilateral species. TH ensure that ontogenetic transitions align with environmental conditions, particularly in terms of energy expenditure, helping animals match their ontogenetic transition with available resources. This alignment spans post-embryonic developmental transitions common to all vertebrates and more subtle adjustments during seasonal changes. The underlying logic of TH function is to synchronize transitions with the environment. This review outlines the fundamental mechanisms of thyroid signalling and shows various ways in which animals use this hormonal system in natural environments. It proposes a model linking TH signalling, environmental conditions, ontogenetic trajectory, and metabolism. TH levels are tightly regulated in adult mammals, as excessively low or high TH levels can have detrimental effects. A negative feedback system allows TH to regulate their own levels through the hypothalamo-pituitary-thyroid (HPT) axis. TH also act as a major effector of seasonality, the process by which species adapt their physiology and reproduction to annual changes in temperature and photoperiod. TH levels are influenced by environmental factors such as resource availability, energy expenditure, thermal adaptation, and salinity acclimation. TH play a critical role in thermogenesis, helping to maintain body temperature and energy expenditure. They are also involved in thermal adaptation, with studies showing that TH levels vary in response to extreme thermic conditions. TH are also involved in salinity acclimation, with studies showing that TH levels are affected by changing salinities in several fish species. TH are involved in seasonal adaptation, with studies showing that TH levels vary in response to seasonal changes. TH are also involved in metamorphosis, a process that is known to be controlled by TH in chordates. TH are key regulators allowing organisms to acclimate to a range of environmental conditions—temperature, salinity, seasons. This role in tailoring physiology to environmental conditions is crucial for metamorphosis. TH are also involved in the regulation of seasonality, the recurring patterns of biological changes that unfold cyclically throughout the year. TH play a critical role in the regulation of seasonality, the recurring patterns of biological changes that unfold cyclically throughout the year. TH are also involved in the regulation of metamorphosis, a process that is known to be controlled by TH in chordates. TH are key regulators allowing organisms to acclimate to a range of environmental conditions—temperature, salinity, seasons. This role in tailoring physiology to environmental conditions is crucial for metamorphosis.