Seasonal influence on the thyroid gland in healthy dogs of various breeds in different weights

The aim of our study was to evaluate the influence of seasonal ambient temperature on the thyroid gland function in terms of serum total thyroxine (TT4), free thyroxine (fT4), total triiodothyronine (TT3), and canine thyroid stimulating hormone (cTSH) concentrations in healthy dogs of various breeds living outdoors and to compare them with those living indoors in flats and houses. The monitoring was conducted for 5 years and our study included 162 indoor dogs and 148 outdoor dogs of both sexes from 2 to 12 years of age, categorised into 3 groups according to their body weight: large (n = 17), medium (n = 16) and small (n = 17) breeds. Comparison of the seasonal serum TT4 and fT4 concentrations in both the indoor and outdoor dogs confirmed their fluctuation in relation to the ambient temperature in all weight groups with the lowest average of TT4 and fT4 concentrations recorded in summer and the highest ones in winter. In dogs kept outdoors, the fluctuation of hormone values was significant (P < 0.05). Seasonal serum cTSH fluctuation was found to be significant (P < 0.05) only in the medium breeds living outdoors, but their concentrations did not exceed the reference ranges. The 5-year monitoring of serum TT3 in indoor and outdoor dogs of large, medium, and small breeds clearly showed that its concentrations were not influenced by varying ambient temperature. The study provides for the first time evidence that serum TT4 and fT4 concentrations in dogs kept outdoors directly depend on ambient temperature. This correlation was also expressed by mathematical equations.