Effects of thyroid status on insulin-like growth factor-I, growth hormone and insulin are modified by food intake

Understanding the interactions between metabolic signals that regulate insulin-like growth factor-I (IGF-I) is crucial to a recognition of mechanisms that control mammalian growth. Thyroid hormones (THs) are essential for normal growth and development, and it has been suggested previously that they can modify circulating IGF-I concentrations. However, the fact that THs influence food intake, which can itself affect plasma IGF-I levels, has been ignored in previous studies. We have therefore investigated the effects of thyroid status on plasma IGF-I under conditions of controlled food intake in young growing pigs. Circulating IGF-I, growth hormone (GH) and insulin levels, were studied in hypo- and hyperthyroid animals on the same level of food intake as euthyroid controls. In addition, a separate group of hyperthyroid animals was given double the amount of food, in order to assess the influence of increased food intake, as would occur naturally in the hyperthyroid state. Hypothyroid animals and hyperthyroids with extra food had the greatest increase in body weight over the 3 weeks of treatment. These two groups had significantly higher circulating IGF-I and insulin concentrations than either the euthyroid or hyperthyroid animals on the same food intake. Integration of GH concentrations from samples taken every 20 min over a 9 h period showed that, by contrast with IGF-I and insulin levels, GH levels were significantly lower in hypothyroids and hyperthyroids on extra food compared with the euthyroids and the hyperthyroids on the same food intake. We conclude that the effects of thyroid status on IGF-I are mediated in part by the effects that THs have on energy balance, and that nutritional signals are capable of modifying the influence of thyroid status per se on circulating IGF-I concentrations.