Cross-Latitudinal, Seasonal and Diurnal Comparisons in Thyroid Hormone Concentrations in Sled Dogs

Problem Statement: Hypothyroidism has been a common disorder in humans and dogs. Amongst a sundry of functions, the thyroid gland plays a key role in metabolism and consequently, hypothyroidism has been a common diagnosis for a decrease in performance in sled dogs. Research has indicated that a variety of environmental factors influence thyroid hormone production, of which, light exposure, climate, latitude, exercise and season demonstrate pronounced effects. Sled dogs are exposed to many of these variables and often display thyroid hormone levels that are clinically below normal ranges. Approach: This study took a cross-latitudinal naturalistic approach in determining the effects of day length, time of day, season, climate and exercise on thyroid hormone function in sled dogs. In the process, appropriate reference ranges for racing sled dogs were established that correlate with other studies involving working dogs. Results: There was a clear indication that thyroid hormones play an integral role in thermoregulation and are greatly affected by environmental cues. Unexpectedly, sled dogs in the sub Arctic were not more prone to hypothyroidism but in fact, had higher levels of most thyroid hormones than dogs residing at lower latitudes. Conclusion: An evolutionary adaptation may account for up-regulation of thyroid function in times of environmental extremes. Consequently, the normal range for most thyroid hormones in sled dogs is lower than clinical standards.