Thyroid Hormone

Thyroxine (T4) is the predominant thyroid hormone in horses but is believed to have less biological activity than triiodothyronine (T3). T3 is important for proper fuel metabolism (glucose absorption, protein synthesis and lipid metabolism), temperature regulation and muscle function. Thyroid hormone concentrations may be influenced by age, season, pregnancy, drugs, stress, stage of training, exercise and nutrition.

 Because thyroid hormone plays a role in the regulation of fuel metabolism and muscle function, a study was conducted to assess the effects of diet on thyroid hormone concentrations in regularly exercised horses. The effects of short-term feed restriction and the calorie source (roughage vs concentrate) on T4 and T3 concentrations and metabolic responses to feeding a small meal and to exercise were studied. Feed restriction was used to simulate the effects of horses going "off feed" for a brief period during a training program.

 Four mature conditioned Thoroughbred geldings were assigned to four treatments in a 4x4 Latin square design, so that every horse received every treatment for 11 or 12 days. The treatments consisted of 1) a nutritionally adequate high roughage ration (70:30% roughage:concentrate) [AHR]; 2) a nutritionally adequate high concentrate ration (40:60% roughage:concentrate) [AHC]; 3) a diet restricted to 70% of 1 [RHR]; 4) a diet restricted to 70% of 2 [RHC].

 On day 9 of each treatment period, each horse was fed 1.0 kg of oats in order to determine thyroid hormone response following a meal. When horses received the AHC diet, serum T4 concentrations were lower compared to when they received the other diets as shown in Figure 1. This result suggests that diet composition can affect the concentration of T4 occurring in serum. Studies in other species have shown that diet can affect the conversion of T4 to T3, thus it is possible that the AHC diet enhanced this conversion. However, average serum T3 concentrations were not affected by diet.

Thyroid hormone response to exercise was evaluated on day 11 or 12 by each horse performing a 25 minute exercise test on a high speed treadmill. Horses performed a standardized exercise test on day 11 or 12 of each period.

When the horses received the RHR diet, T4 concentrations were elevated during the exercise test and recovery periods. This response may again indicate an effect of diet on regulation of conversion of T4 to T3. However, T3 concentrations were not affected by diet but tended to increase with exercise. The performance variables of lactate concentration, heart rate and rectal temperature were not affected by diet. Further study is underway to evaluate the effects of longer term diet manipulation on T4 and T3 concentrations.

 From the above results, T4 concentration was lower after a meal when horses received the AHC diet, which may suggest that the high concentrate diet increased T4 to T3 conversion thus resulting in a lower serum T4 concentration. When the horses received the RHR diet, T4 concentration was elevated throughout exercise which may imply decreased conversion of T4 to T3 thus resulting in a higher serum T4 concentration.

 In practice, blood samples are often collected at the most convenient time without regard to feeding or exercise condition. From the above study, feed restriction and calorie source appear to affect total serum T4 concentrations but have little influence on total serum T3 concentrations in healthy adult geldings undergoing conditioning. These results indicate that exercise, diet and feeding state should be considered when total T4 concentration in a serum sample is being used to assess thyroid states in horses.

Debra M. Powell, (606) 257-7510, or
Dr. Laurie Lawrence,, (606) 257-7509,
Department of Animal Sciences, University of Kentucky