Effect of fasting on thyroxine, 3,5,3'-triiodothyronine, and cortisol concentrations in serum of dogs

The present study was designed to compare basal and stimulated concentrations of 3,5,3'-triiodothyronine (T3), thyroxine (T4), and cortisol in serum of dogs fasted 12 or 18 hours (to represent overnight fasting) or 24 or 36 hours (to represent prolonged inappetence) with those of dogs that were not fasted. Twenty-five adult Beagle bitches were allotted to 5 experimental fasting groups (0, 12, 18, 24, and 36 hours). Blood samples for hormonal analyses were obtained 4, 3, 2, and 1 hour before food was removed; at the time of food removal; 1 hour after food was removed; and every 2 hours during experimental fasting until 0800 hours on the day fasting ended. Dogs were injected with 5 IU of thyrotropin, IV, and 2.2 IU of adrenocorticotropin/kg, IM, to evaluate thyroidal and adrenocortical endocrine reserves. Additional blood samples were collected 0.5, 1, 2, 3, and 4 hours after injections were given. Serum concentrations of T3, T4, and cortisol were determined by validated radioimmunoassays. Body weights and ages of the dogs and food consumption during a 2-hour preliminary feeding period before dogs were fasted did not differ among fasting groups. Length of fasting did not affect serum concentrations of T3 or T4 in dogs at 12, 18, 24, or 36 hours after food was removed. Mean serum concentrations of cortisol in dogs fasted 12 or 24 hours were lower than those in dogs that were not fasted. Serum concentrations of the hormones after thyrotropin and adrenocorticotropin were injected were not affected by fasting.(ABSTRACT TRUNCATED AT 250 WORDS)