Semi-arid conditions can adversely affect livestock productivity and change certain physiological parameters. The relationship between hair coat and thyroxine levels in Morada Nova ewes was evaluated through environmental factors, such as air temperature, relative air humidity, radiant and Black Globe Humidity Index to gain a better understanding of thermoregulation mechanisms in these animals. Measurements were taken from July to January in 383 Morada Nova ewes. The variables studied included rectal temperature (RT), respiratory rate (RR), coat thickness (CT), hair length (HL), hair diameter (HD), hair density (D) and thyroid hormones. The data underwent multivariate statistical analyses and a significant inverse relationship was found between coat traits and Thyroxine (T4). The animals that exhibited greater HL, coat density and CT showed lower T4 concentrations. Coat traits showed a strong interaction with physiological mechanisms and can be considered relevant in maintaining homeostasis. Hair traits play an important role in this process, since T4 reduction was stronger in animals that showed difficulties in eliminating heat, which were the ones that had greater HL, hair density and HD. By contrast, animals with a hair coat more favourable to heat losses had higher levels of thyroid hormones.