Thyroid hormone-induced changes in body temperature and metabolism during exercise in dogs

Thyroid and Lipid Status in Guide Dogs During Training: Effects of Dietary Protein and Fat Content

Nutrition plays a leading role that most influences thyroid response and energetic metabolism. Aim was to compare the effect of diet on thyroid and lipid status in guide dogs during a 12-weeks training period. Eight Labrador Retrievers were divided into two groups homogeneous for sex, age, body weight, and Body Condition Score (BCS) and fed two commercial diets one, HPF, characterized by low-carbohydrate/high-protein/high-fat (29%:39%:19% as-fed) and the other, LPF, by high-carbohydrate/low-protein/low-fat (50%:24%:12% as-fed) content. The serum thriiodothyronine (T₃), thyroxine (T₄), cholesterol (CHOL), triglycerides (TAGs) and non-esterified fatty acids (NEFA) were determined at Day 0, 28, 56, and 84, before the daily training. Statistical model included the effects of Diet (HPF vs. LPF) and Time (Day 0 to Day 84), and their interaction. In the HPF group, Diet significantly (p < 0.01) increased T₄, CHOL, and TAGs and decreased NEFA. In both groups, Time significantly (p < 0.05) increased T₄ and TAGs, CHOL at Day 28, and NEFA at Day 56. The interaction did not influence serum hormones and lipid pattern. The adjustments in thyroid and lipid responses to moderate exercise in HPF group were driven mainly by the nutrient composition of the diet in relation to the involvement of metabolic homeostasis.

Plasma free fatty acids in neonates with congenital hypothyroidism

Since the introduction of neonatal mass screening for congenital hypothyroidism (CH), numerous cases have been detected. It is of interest that even severely hypothyroid neonates rarely exhibit bradycardia, hypothermia, or inactivity, which have been recognized as typical signs of CH. Regarding neonates and young infants, few reported data are available on the effects of thyroid hormones on energy expenditure. Plasma free fatty acids (FFAs), markers for lipolysis, play essential roles in maintaining physiologic homeostasis. To study fuel utilization in CH neonates, we measured heart rates, plasma FFA, and thyroid hormones before and after levothryoxine (LT4) replacement therapy. Fifty-five screen-detected CH neonates and 29 age-matched normal neonates for controls were enrolled. The CH neonates were divided into two groups according to serum thyroid hormone levels: a mildly hypothyroid group (n = 37), serum thyrotropin (TSH) less than 100 microIU/mL and free thyroxine (FT4) 0.6 ng/dL or more; and a severely hypothyroid group (n = 18), TSH 100 microIU/mL or more and FT4 less than 0.6 ng/dL. Twenty-four of the 55 patients had their heart rates measured by electrocardiography. Fasting blood samples were taken from the subjects during physical movements. Serum levels of TSH, FT4, FFA, and other blood chemicals, measured on an autoanalyzer system in our hospital, were compared before and after LT4 substitution therapy. The following results were obtained. The mean plasma FFA values before LT4 replacement were 208.5 +/- 89.4 microEq/L in the mildly hypothyroid group, 228.5 +/- 114.7 microEq/L in the severely hypothyroid group, and 213.9 +/- 97.7 microEq/L in controls. No statistical differences were noted among the three values. Two months after LT4 replacement therapy, at the age of 3 months, plasma FFA concentrations significantly increased in both groups compared with those before the therapy. Control infants also showed a significant increase in plasma FFA concentrations from 1 to 3 months of age. There were no significant differences in plasma FFA concentrations among the three groups at the age of 3 months. No significant correlations were found between plasma FFA and serum thyroid hormones. From these results it is suggested that in neonates and young infants, thyroid hormones do not play major roles in mobilization of fats through the adrenergic regulation of lipolysis for energy supply. This may be one of the reasons for the unexpectedly mild signs and symptoms in the screen-detected hypothyroid neonates.

Low efficiency of oxygen utilization during exercise in hyperthyroidism

STUDY OBJECTIVE

The mechanism of exercise intolerance in hyperthyroidism has not been fully elucidated. This study was undertaken to determine if hyperthyroidism reduced the efficiency of sub-maximal exercise.

STUDY DESIGN

We measured cardiorespiratory variables up to the anaerobic threshold (AT) during ramp-loading cycle ergometry in 12 patients (New York Heart Association functional class II or III). Studies were performed in the hyperthyroid state and repeated in the euthyroid state after 10 months of medical treatment. In 10-W steps from rest to the AT, we measured oxygen uptake (VO2) as a measure of total body work rate, and pressure rate product (PRP) as a measure of cardiac work rate. Loading watts at AT divided by the increment of Vo2 from rest to the AT (delta Watt/delta VO2) was calculated as an index of work efficiency (where delta means the increment of each value from rest to the AT).

RESULTS

VO2 and PRP at the AT were not significantly different between hyperthyroid and euthyroid states (VO2, 16.6 +/- 3.0 vs 17.5 +/- 2.3 mL/min/kg; PRP, 229 +/- 41 vs 218 +/- 28 x 10(2) mm Hg/min). However, loading watts at the AT were significantly lower in the hyperthyroid than the euthyroid state (28 +/- 22 vs 60 +/- 14 W: p < 0.01). VO2 and PRP while hyperthyroid were significantly higher than when euthyroid at every 10-W step during ramp-loading exercise. Furthermore, delta Watt/delta VO2 was significantly lower in hyperthyroid than euthyroid states (p < 0.001). There was a significant inverse correlation-ship between triiodothyronine and delta Watt/delta Vo2 (r = -0.654, p < 0.001).

CONCLUSION

Hyperthyroidism causes low work efficiency, which may limit exercise tolerance.

Changes in body temperature after administration of antipyretics, LSD, delta 9-THC, CNS depressants and stimulants, hormones, inorganic ions, gases, 2,4-DNP and miscellaneous agents

This survey concludes a series of complications of data from the literature, primarily published since 1965, on thermoregulatory effects of antipyretics in afebrile as well as in febrile subjects, LSD and other hallucinogens, cannabinoids, general CNS depressants, CNS stimulants including xanthines, hormones, inorganic ions, gases and fumes, 2,4-dinitrophenol and miscellaneous agents including capsaicin, cardiac glycosides, chemotherapeutic agents, cinchona alkaloids, cyclic nucleotides, cycloheximide, 2-deoxy-D-glucose, dimethylsulfoxide, insecticides, local anesthetics, poly I:poly C, spermidine and spermine, sugars, toxins and transport inhibitors. The information listed includes the species used, route of administration and dose of drug, the environmental temperature at which the experiments were performed, the number of tests, the direction and magnitude of body temperature change and remarks on the presence of special conditions such as age or lesions, or on the influence of other drugs, such as antagonists, on the response to the primary agents.

Thyroid hormones and lipolysis in physically trained rats

In rats a single bout of exercise resulted in increased triiodothyronine (T3), thyroxine (T4), and triiodothyronine/reverse triiodothyronine (T3/rT3) ratio 20 hr after exercise. The effect of norepinephrine on lipolysis in vitro was potentiated. In trained rats no changes were found in T4, T3, or rT3 concentrations. The T3/rT3 ratio as well as basal and stimulated TSH concentrations decreased in comparison with sedentary, freely eating rats. Moderate food restriction to produce a body weight similar to that of trained animals caused no changes in T4, T3, or rT3 concentrations but caused a decrease in T3/rT3 and in TSH levels. Training and moderate food restriction groups were not different. T3 in vitro caused a potentiation of catecholamine induced lipolysis in trained and food-restricted animals. With aging the serum concentration of T3 decreased and that of rT3 increased. Acute and chronic exercise both exert an effect on peripheral hormonal responses of lipolysis, while they have different and opposite effects on thyroid hormone concentrations. Physical training seems to have effects in this regard similar to those of moderate energy intake restriction. The results suggest that changes in peripheral effects of thyroid hormones during training should attract more attention.

Metabolic and temperature responses to physical exercise in thyroidectomized dogs

The purpose of the present work was to further elucidate the role of thyroid hormones in the control of body temperature and metabolism during physical exercise. Changes in rectal temperature (Tre), some parameters of exercise-metabolism and in the plasma noradrenaline (NA) levels were examined in eight dogs performing submaximal treadmill exercise to exhaustion before and after thyroidectomy (THY). The metabolic 'responses to adrenaline (A) infusion were also compared in intact and THY dogs. During the exercise performed by THY dogs Tre increases were markedly attenuated, plasma FFA level increases were reduced and the pattern of plasma NA changes was modified in comparison with control runs. The reduced exercise-induced FFA mobilization in THY dogs might be attributed to a lower activation of the adrenergic system in the later stage of exercise and to the weaker lipolytic action of catecholamines. The attenuated Tre increases during exercise performed by THY dogs and the exercise-hyperthermia described previously in dogs treated with thyroid ormones suggest that an optimum level of thyroid hormones is necessary to induce typical changes in body temperature during physical exercise.

Metabolic responses to catecholamines in dogs injected with a single dose of triiodothyronine

Lipolytic and glycogenolytic responses to catecholamine infusions were studied in resting dogs before and 20 h following administration of a single dose (0.1 mg/kg) of triiodothyronine (T3). In the dogs pretreated with T3 much higher increases in the plasma FFA concentration were found both during noradrenaline and adrenaline infusions in comparison with control experiments. Adrenaline-induced increases in blood LA and glucose levels were also significantly higher in T3-pretreated dogs than in controls. The blockade of beta-adrenergic receptors with propranolol prevented the increases in blood FFA and LA concentrations during subsequent adrenaline infusion. Phentolamine -- the alpha-adrenergic blocking agent -- infused to the T3-pretreated dog inhibited the adrenaline-induced rise in blood glucose level. The observed changes in the metabolic responses to catecholamines induced by triiodothyronine pretreatment indicate that at least in the dog this hormone potentiates both the lipolytic and glycogenolytic effects of catecholamines acting on appropriate adrenergic receptors.