Seasonal thyroid hormone alterations in Kermani sheep: An adaptive thermoregulatory mechanism for heat stress survival

Elevated ambient temperatures adversely affect livestock production. Among domestic animals, sheep in southern Iran experience heat stress for seven months annually based on the Temperature-Humidity Index (THI), sheep in this region experience heat stress conditions for seven months of the year. Understanding the endocrine mechanisms that regulate metabolic homeostasis may provide insights into how sheep cope with such stress. Thyroid hormones—namely triiodothyronine (T3) and thyroxine (T4)—play fundamental roles in metabolic processes and thermoregulation. In this study, plasma levels of T3 and T4 were measured in Kermani sheep during winter and summer. Both hormones were significantly lower in summer than in winter (P < 0.007). However, the calculated T3/T4 ratio was significantly higher in summer (P < 0.001), indicating a 15-fold increase in the conversion of T4 to T3 compared to winter. Although overall thyroid hormone production declined during summer, Although overall thyroid hormone concentrations declined during summer, the increased T3/T4 ratio suggests possible alterations in peripheral thyroid hormone metabolism and conversion, which may contribute to the adaptive response to high ambient temperatures. Further studies measuring rT3 levels and deiodinase activity are required to confirm the underlying mechanisms. In conclusion, seasonal modulation of thyroid hormone dynamics represents a key thermoregulatory strategy in Kermani sheep.