Thyroid Hormone Metabolism After Acclimatization to a Warm or Cold Temperature Under Conditions of High or Low Energy Intake

Abstract

SUMMARYThe separate effects of ambient temperature and energy intake on the metabolism of thyroid hormones in the young pig have been investigated. Piglets were kept singly from 4 weeks of age at 10 or 35 'C on a high (H), or low (L) energy intake, such that H = 2L. At the tenth week of age blood samples were taken 16-21 h after the last meal for the determination of plasma concentrations ofthyroxine (T4) and 3,5,3'-triiodothyronine (T3). At the end of the tenth week of age the fractional disappearance rate (K) of T4 was determined and finally the K value for T3 was estimated.Plasma concentrations of T4 and T3 were greater in animals on a high intake than in those on a low intake, (P < 0 02), but the temperature at which the animals lived had no significant effect. The values of K for T4 and T3 were greater at 10 than at 35 IC (P < 0 010 and P < 0 05), and energy intake had no significant effect. Calculations ofthe catabolic rates in nmol/kg. h revealed that for T4 the rate was greater at 10 than at 35 IC (P < 0 001) and on a high than on a low intake (P < 0 01). Similar differences were found between the mean values of T3 with respect to both temperature and energy intake, but the variance was large and they did not reach statistical significance.It is concluded that the metabolism of thyroid hormones is influenced independently by both ambient temperature and energy intake.

Thyroid Hormone Nuclear Receptors in Skeletal Muscle as Influenced by Environmental Temperature and Energy Intake

“…One of the most obvious actions of thyroid hormones is on the resting metabolic rate at thermal neutrality which is known to depend, at least in part, on the number of Na+-K+ pumps (Ismail-Beigi & Edelman, 1970). The resting metabolic rates and body temperatures of pigs of the kind observed in the present study were measured by Macari, Ingram & Dauncey (1983 b). Deep body temperature was lower for animals at 10 than at 35°C, particularly for those on the low energy intake.…”

“…In the present study the loss of receptors cannot have been mediated by such a mechanism, since it is known that in animals of the kind which have been studied it is those on the high energy intake which have the highest plasma concentrations ofT3 (Macari et al 1983 a). Moreover, in the rat the number of receptors in the liver were found to be similar in hypo-, hyper-and euthyroid animals (Spindler et al 1975;DeGroot, Torresani, Carrayon & Tirard, 1976) and as yet no down-regulation of receptors in response to a high plasma concentration of T3 has been demonstrated in vivo for any tissue (Samuels, 1983).…”

“…The rates of metabolism of thyroid hormones and their plasma concentrations are also modified by the animal's energy supply, and the time since the last meal in pigs (Ingram & Dauncey, 1980;Ingram & Ramsden, 1981;Dauncey, Ramsden, Kapadi, Macari & Ingram, 1983b) and by food deprivation in man (O'Brien, Bybee, Burman, Osburne, Ksaizek, Wartofsky & Georges, 1980;Utiger, 1982). In the pig both environmental temperature and energy intake have been shown to have independent effects on the metabolism of the thyroid hormones either through their effects on the fractional disappearance rate, or on plasma concentration (Macari, Dauncey, Ramsden & Ingram, 1983 a). Moreover, at thermal neutrality the effect of energy intake is modified after several weeks (Griggio & Ingram, 1985).…”

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

“…The hyperthyroid animals on 3% and 6% food intakes had circulating T 4 levels which were on average 125% and 87% higher than the euthyroid control group. These trough levels of T 4 in the hyperthyroid animals were at least 40% higher than the physiologically elevated levels observed in animals of the same breed and age on a high food intake or after acclimation to a cold ambient temperature (14,27). Table 1 also gives the circulating T 3 concentrations in the four groups of animals.…”

“…A direct correlation exists between energy intake and circulating levels of THs (13,14) and IGF-I (15)(16)(17), whereas undernutrition leads to an elevation in circulating GH (18). The precise mechanisms by which energy intake modifies circulating IGF-I concentrations are unclear, although they may involve changes in circulating thyroid and pancreatic hormone levels (19).…”