Studies on the metabolism of adipose tissue. XX. The effect of thyroid status upon oxygen consumption and lipolysis

The effect of growth hormone replacement on the thyroid axis in patients with hypopituitarism: in vivo and ex vivo studies

OBJECTIVE

Alterations in the hypothalamic-pituitary-thyroid axis have been reported following growth hormone (GH) replacement. The aim was to examine the relationship between changes in serum concentration of thyroid hormones and deiodinase activity in subcutaneous adipose tissue, before and after GH replacement.

DESIGN

A prospective, observational study of patients receiving GH replacement as part of routine clinical care.

PATIENTS

Twenty adult hypopituitary men.

MEASUREMENTS

Serum TSH, thyroid hormones - free and total thyroxine (T4) and triiodothyronine (T3) and reverse T3, thyroglobulin and thyroid-binding globulin (TBG) levels were measured before and after GH substitution. Changes in serum hormone levels were compared to the activity of deiodinase isoenzymes (DIO1, DIO2 and DIO3) in subcutaneous adipose tissue.

RESULTS

The mean daily dose of growth hormone (GH) was 0·34 ± 0·11 mg (range 0·15-0·5 mg). Following GH replacement, mean free T4 levels declined (-1·09 ± 1·99 pmol/l, P = 0·02). Reverse T3 levels also fell (-3·44 ± 1·42 ng/dl, P = 0·03) and free T3 levels increased significantly (+0·34 ± 0·15 pmol/l, P = 0·03). In subcutaneous fat, DIO2 enzyme activity declined; DIO1 and DIO3 activities remained unchanged following GH substitution. Serum TSH, thyroglobulin and TBG levels were unaltered by GH therapy.

CONCLUSIONS

In vitro analysis of subcutaneous adipose tissue from hypopituitary human subjects demonstrates that GH replacement is associated with significant changes in deiodinase isoenzyme activity. However, the observed variation in enzyme activity does not explain the changes in the circulating concentration of thyroid hormones induced by GH replacement. It is possible that deiodinase isoenzymes are differentially regulated by GH in other tissues including liver and muscle.

Serum irisin levels and thyroid function--newly discovered association

Irisin is a newly discovered adipo-myokin, which is reported to have a significant influence on the body metabolism and thermogenesis. Other influencing factors on metabolic state are thyroid hormones, which increase heat production and control the energy balance. Due to numerous similarities in action it seems imperative to explore these substances' potential mutual influence on the body. The aim of the study is to provide the first ever, according to our knowledge, evaluation of serum irisin concentrations in patients with thyroid dysfunction and its correlation with creatine kinase (CK) levels - a serum marker of muscle damage. The studied group consisted of 20 patients with newly diagnosed thyroid disorder - hyperthyroidism or hypothyroidism. Venous blood samples were analyzed for irisin, thyrotropin (TSH), free thyroxine (FT4), free triiodothyronine (FT3) and CK serum concentrations. Patients' body mass index (BMI), body weight and muscle mass were evaluated using bioelectrical impedance analysis (BIA). Median serum irisin levels were lower in hypothyroid (117.30ng/ml) than hyperthyroid (161.95ng/ml) patients with a borderline statistical significance (p=0.0726). The negative correlation between irisin and TSH levels was demonstrated (r=-0.4924, p=0.0230), as well as the positive correlation between irisin and FT4 levels (r=0.4833, p=0.0360). The CK level was negatively correlated with irisin, FT4 and FT3 concentrations (r=-0.7272, p=0.0140; r=-0.9636, p=<0.0001; r=-0.8838, p=0.0007, respectively). The study demonstrates that irisin concentrations may vary according to the thyrometabolic state, which potentially could be related to the degree of muscle damage.

Hypoglycaemic coma in severe primary hypothyroidism

A 76-year-old woman was admitted to the hospital in comatose condition. Her blood glucose was 1.7 mM. Immediately after intravenous glucose treatment she attained normal consciousness. The diagnosis of severe primary hypothyroidism was subsequently made and no sign of other diseases was detected. After thyroid replacement therapy fasting blood glucose levels rose to normal and no further hypoglycaemic episodes occurred. It is emphasized, that hypoglycaemia may be the direct cause of severely impaired consciousness in hypothyroidism requiring immediate and specific therapy.

Do glucocorticoids cause spinal epidural lipomatosis? When endocrinology and spinal surgery meet

Here, we report pathogenetic aspects of spinal epidural lipomatosis (SEL) based on a literature review. SEL is a rare entity but can cause significant morbidity. Its symptoms can be identical to those of more common disorders such as vertebral and disc disease, and cord lesions (for example, transverse myelitis, multiple sclerosis and syringomyelia). Therefore, it often goes undiagnosed. In addition, SEL occurs in patients on glucocorticoid therapy, which can lead to myopathy, thereby mimicking the motor symptoms of SEL. Glucocorticoids seem to play a major role in the development of SEL, although idiopathic SEL has also been reported. The latter occurs almost exclusively in obese individuals who may have concurrent hypercortisolism. Once clinically suspected, SEL is best diagnosed by magnetic resonance imaging (MRI). Treatment of SEL is directed at reducing body weight in patients with idiopathic SEL, and at decreasing glucocorticoid excess in patients with endogenous or exogenous hypercortisolism. In severe cases, decompressive laminectomy might become necessary to alleviate the neurological symptoms caused by spinal cord compression.

Effect of short- and long-term experimental hyperthyroidism on plasma glucose level and insulin secretion during an intravenous glucose load and on insulin binding, insulin receptor kinase activity, and insulin action in adipose tissue

Glucose disposal, insulin secretion, and insulin action in adipose tissue were measured in rats treated for 10 or 30 days with high doses of thyroxine (T4). Acutely induced hyperthyroidism produced a high rate of glucose disposal after an intravenous glucose tolerance test (IVGTT), accompanied by a high glucose-stimulated insulin secretion. In addition, in these rats the following phenomena were observed: (1) high insulin binding to isolated adipocytes due to an increase in the insulin receptor number; (2) high insulin binding to partially purified fat insulin receptors; (3) normal tyrosine kinase activity of fat insulin receptors; and (4) high insulin action in isolated adipocytes, such as glucose transport and lipogenesis. Chronically induced hyperthyroidism produced high rates of glucose disposal after an IVGTT, accompanied by an increase of basal and glucose-stimulated insulin secretion. These rats showed (1) normal insulin binding to either isolated adipocytes or partially purified insulin receptors; (2) normal tyrosine kinase activity of fat insulin receptors; (3) normal insulin action in isolated adipocytes. In conclusion, exogenous hyperthyroidism induced an increase in glucose disposal, probably due in part to high insulin secretion. In short-term T4-treated rats an additional increase of insulin action in adipocytes was also observed.

Effect of thyroid hormone status on the expression of the mRNAs of components of the lipolytic regulatory cascade in brown adipose tissue

1. The levels of mRNAs for RII beta and G beta were about 50% lower in brown adipose tissue (BAT) from hyperthyroid than from hypothyroid rats. 2. Treatment of hypothyroid rats with T3 resulted in a 50% decrease in mRNAs for RII beta and G beta in BAT occurring by 12 hr after treatment. 3. The levels of mRNAs for hormone-sensitive lipase, G alpha s and C alpha in BAT were unchanged by thyroid hormone status. 4. The results suggest that thyroid hormone may be involved in negative regulation of the expression of RII beta and G beta at the transcriptional level in BAT.

Graves exophthalmos unrelated to extraocular muscle enlargement. Superior rectus muscle inflammation may induce venous obstruction

Exophthalmos is the most commonly measured sign of Graves ophthalmopathy, whereas enlargement of the extraocular muscles is the principal pathologic abnormality. The purpose of this article is to explore possible etiologies of increased volume of orbital fat and of proptosis in patients with no substantially increased total extraocular muscle volume. Computed tomographic scans of the 13 orbits reviewed in this study had the following characteristics in common: a fine, reticular pattern within the orbital fat, a prominent superior ophthalmic vein, and an enlarged superior rectus muscle. Quantitative analysis revealed that superior rectus muscle volume showed a statistically significant correlation with proptosis, whereas medial, lateral, and inferior rectus muscle volumes did not correlate with proptosis. Based on anatomic considerations, the authors postulate that superior rectus muscle enlargement alone may produce reduced venous outflow from the orbit, thereby expanding the apparent orbital fat volume and producing proptosis.

Functional relationship of thyroid hormone-induced lipogenesis, lipolysis, and thermogenesis in the rat

Metabolic balance studies were carried out to determine the interrelationships of thyroid hormone-induced lipogenesis, lipolysis, and energy balance in the free-living rat. Intraperitoneal doses of 15 micrograms triiodothyronine (T3)/100 g body wt per d caused an increase in caloric intake from 26.5 +/- 1.7 (mean +/- SEM) kcal/100 g per d to 38.1 +/- 1.5 kcal/100 g per d. Food intake, however, rose only after 4-6 d of treatment and was maximal by the 8th day. In contrast, total body basal oxygen consumption rose by 24 h and reached a maximum by 4 d. Since total urinary nitrogen excretion and hepatic phosphoenolpyruvate carboxykinase mRNA did not rise, gluconeogenesis from protein sources did not supply the needed substrate for the early increase in calorigenesis. Total body fat stores fell approximately 50% by the 6th day of treatment and could account for the entire increase in caloric expenditure during the initial period of T3 treatment. Total body lipogenesis increased within 1 d and reached a plateau 4-5 d after the start of T3 treatment. 15-19% of the increased caloric intake was channeled through lipogenesis, assuming glucose to be the sole substrate for lipogenesis. The metabolic cost of the increased lipogenesis, however, accounted for only 3-4% of the T3-induced increase in calorigenesis. These results suggest that fatty acids derived from adipose tissue are the primary source of substrate for thyroid hormone-induced calorigenesis and that the early increase in lipogenesis serves simply to maintain fat stores. Since the mRNAs coding for lipogenic enzymes rise many hours before oxygen consumption and lipolysis, these results suggest that T3 acts at least in part by an early coordinate induction of the genes responsible for these processes.

Spinal epidural lipomatosis: report of a case secondary to hypothyroidism and review of literature

Epidural lipomatosis usually occurs as a complication of steroid therapy. This report includes the first description of a case secondary to hypothyroidism. Adults in all age groups can be affected and present with progressive signs and symptoms of spinal cord or cauda equina compression. The dorsal aspect of the thoracic spine is most commonly involved, usually in its total extent. Spine radiographs are normal and myelography poorly delineates the nature and extent of abnormality. While computed tomography can provide information about the nature of the spinal mass, magnetic resonance imaging is the investigation of choice. Extensive decompressive laminectomy is useful but the results are variable. Correction of underlying endocrine abnormality can reverse the process. The need for early diagnosis of this disorder is therefore emphasized.

Thermogenic and metabolic consequences of thyroid hormone treatment in brown and white adipose tissue

Male rats were treated with triiodothyronine in the drinking water for 12 days. In vitro rates of isoprenaline stimulated lipolysis were significantly greater in brown but not white adipose tissue. Rates of [14C]glucose incorporation into triacylglycerols were significantly reduced in BAT (brown adipose tissue) and WAT (white adipose tissue) under basal and isoprenaline stimulated conditions. In a second experiment, hyperthyroid animals showed impaired weight gain, despite increased food intake during 19 days' treatment. Energy expenditure on days 5 and 12, and BAT core temperature differences (TBAT - TCORE) on day 19, were significantly greater than in control animals. Epididymal white fat pad weight was reduced and interscapular brown fat pad weight increased by triiodothyronine treatment.

Nuclear triiodothyronine binding sites in rat adipocytes

Most effects of thyroid hormones appear to be mediated by the binding of triiodothyronine (T3) to nuclear triiodothyronine binding sites (NT3BS). Although thyroid hormones influence adipocyte metabolism, NT3BS have not been described in mature adipocytes yet. This report describes T3 nuclear binding in isolated nuclei from rat epididymal fat pad adipocytes. Nuclei were isolated by exposing collagenase-dispersed adipocytes to STM (sucrose, 0.25 mol/L; TRIS, 20 mmol/L; MgCl2, 1.1 mmol/L, pH 7.85) containing 0.5% (vol/vol) Triton X-100. Incubation of nuclei suspended in STM/EDTA (2 mmol/L)/DTT (5 mmol/L) with 125I-T3 and varying concentrations of unlabeled T3 at 37 degrees C for one hour revealed the presence of high-affinity, low-capacity NT3BS. Their MBC was 0.39 +/- 0.04 (SD) ng of T3 per milligram of DNA and their Kd was 1.4 +/- 0.5 (SD) X 10(-10) mol/L T3. Specific binding reached a plateau between 30 minutes and two hours of incubation. The addition of 5 X 10(-7) mol/L T3 to nuclei incubated for one hour with 2 X 10(-11) mol/L T3 completely displaced the specifically bound 125I-T3 within 30 minutes. Thyroxine (T4) and 3, 3', 5'-triiodothyronine (rT3) could displace 125I-T3 from the NT3BS but were less than 10% and 1% as effective, respectively, as T3. Rat epididymal fat pad adipocytes contain NT3BS, the binding characteristics of which are similar to those of rat hepatic NT3BS.

Direct effect of triiodothyronine on glucose utilization in adipocytes from obese and nonobese Zucker rats

The direct effects of L-T3 (triiodothyronine) on glucose utilization, 2-deoxyglucose uptake and O2 consumption were evaluated in vitro in isolated adipocytes from 6-week-old obese and nonobese Zucker rats. Adipocytes treated for 30 min with L-T3 had a 20-25% decrease in glucose conversion to CO2, fatty acids and glyceride-glycerol and uptake of 2-deoxyglucose. Incubation of adipocytes with either D-T3 or rT3 (reverse T3) did not decrease utilization of glucose. T3 also decreased the insulin-stimulated fatty acid synthesis from glucose in both obese and nonobese adipocytes. Adipocyte O2 consumption was decreased by T3 in media that contained glucose but not in media that contained either acetate or no exogenous substrate. These data indicate that T3 has a direct, immediate and specific action on adipocyte glucose utilization and may act to modulate adipose lipogenesis and lipolysis.

Diurnal hormone-metabolite profiles in hypothyroidism

To investigate the influence of thyroid hormones on intermediary metabolism in man, hormone and metabolite profiles were obtained over a 12-h period of normal meals and activity in eight hypothyroid subjects before and during thyroxine replacement therapy, and in sixteen matched controls. The fasting blood glucose concentration and the mean 12-h blood glucose concentration were normal in hypothyroid subjects but the blood glucose response to breakfast was exaggerated. Fasting blood lactate and pyruvate levels were normal but post-prandial hyperlactataemia and hyperpyruvicaemia were found and mean 12 h values for lactate (hypothyroid 1.80 +/- 0.06 v. control 0.77 +/- 0.03 mmol/l, P less than 0.01) and pyruvate (0.10 +/- 0.01 v. 0.08 +/- 0.003 mmol/l, P less than 0.01) were elevated. Blood alanine concentrations were elevated only in the evening. Although plasma non-esterified fatty acid levels were normal, fasting blood glycerol levels were decreased (0.06 +/- 0.01 v 0.08 +/- 0.01 mmol/l, P less than 0.001) and this decrease persisted throughout the 12-h period. Blood total ketone body concentrations did not differ from controls, but, as for plasma NEFA and blood glycerol, the normal preprandial rise in concentration was absent. Serum insulin, glucagon and growth hormone concentrations did not differ from control values at any time. Six months of thyroxine (T4) treatment produced a rise in blood glycerol concentration (mean 12 h value during T4 therapy, 0.06 +/- 0.01; before T4 therapy, 0.04 +/- 0.005 mmol/l; P less than 0.01) but not to control values (0.08 +/- 0.01 mmol/l). Concentrations of glucose and other gluconeogenic precursors were unaltered by therapy but the insulin response to meals and the mean 12 h serum insulin concentration were increased.

Influence of hyperthyroidism on splanchnic exchange of glucose and gluconeogenic precursors

Arterial concentrations and splanchnic exchange of glucose, amino acids, lactate, pyruvate, and glycerol were determined in 14 hyperthyroid patients and 12 healthy controls. Seven of the patients were restudied after 5-12 mo of medical management at which time there was chemical and clinical evidence of a euthyroid state. The arterial level of glucose was slightly higher (+10%) in the patient group and the glycerol concentration was three times greater among the patients. The plasma levels of the glycogenic amino acids, alanine, glycine, and serine were decreased by 20-30%, while the concentrations of leucine, isoleucine, and tyrosine were increased by 20-80%. The levels of lactate and pyruvate were similar in patients and controls as were insulin and glucagon concentrations. Splanchnic glucose output in the patient group was 35% lower than in controls. However, total splanchnic uptake of glucogenic precursors was 100% higher than in controls and showed a direct linear correlation with serum triiodothyronine. Total precursor uptake could account for 75% of splanchnic glucose output in the patients, compared to 26% in controls. The increase in uptake of lactate, alanine, and other amino acids was due to a 35-80% rise in splanchnic fractional extraction plus a 20% rise in estimated hepatic blood flow. When the patients were restudied after medical treatment splanchnic exchange of glucose and glucose precursors had reverted to normal values. The present findings demonstrate that in hyperthyroidism (a) total splanchnic glucose output is reduced in relation to controls, (b) splanchnic uptake of gluconeogenic precursors is accelerated, largely due to a rise in fractional extraction of precursor substrates and to a smaller extent, as a result of an increase in hepatic blood flow, and (c) these changes revert to normal when a euthyroid state has been achieved.

Metabolic aspects of the calorigenic effect of thyroid hormone in mammals

The increased BMR in hyperthyroidism may be accounted for by the use of chemical energy for metabolic processes and work. Major contributors are the heart work and futile cycling of FFA into triglyceride in adipose tissue, whereas gluconeogenesis in liver and the maintenance of sodium and potassium concentration gradients across the plasma membranes are unlikely to play any significant role. Information on the use of energy for protein turnover and urea production is lacking. The rate of oxygen uptake is not increased in the brain, spleen and testis. The main metabolic fuel seems to be free fatty acid. The mechanism which enables the hyperthyroid tissue to maintain normal concentrations of ATP, ADP and inorganic phosphate, (Pi) despite an increased turnover of energy-rich phosphates, is not fully elucidated. However, ultimately the increased rate of oxygen uptake in hyperthyroidism seems to rely upon an increased capacity for the transport of cytosolic ADP and Pi into mitochondria. The transport capacity is increased by an increased area of the mitochondrial membrane per g tissue and by a change in the kinetics of translocation of substrates for oxidative phosphorylation. Other transport processes across the mitochondrial membrane are also changed by hyperthyroidism, e.g. long chain fatty acid transport via carnitine acyl transferase, and oxaloacetate transport via substrate shuttles.

Adrenergic modulation of insulin secretion in vivo dependent on thyroid states

Insulin secretory responses via adrenergic mechanisms were studied in vivo with hyperthyroid rats prepared by daily injections with thyroxine and with rats rendered hypothyroid by the addition of methylthiouracil in the drinking water. Isoproterenol, a beta-adrenergic agent, caused hyperinsulinemia in hyperthyroid rats more markedly than in euthyroid rats, but failed to induce hyperinsulinemia in hypothyroid rats. The isoproterenol-induced hyperinsulinemia was abolished by a beta-adrenergic receptor blocker in hyperthyroid as well as in euthyroid rats. The glucose-induced hyperinsulinemia was enhanced in hyperthyroid, was not essentially affected in euthyroid, and was inhibited in hypothyroid rats, by an alpha- and beta-adrenergic agent such as epinephrine. It is concluded that the relative function of alpha- to beta-adrenergic receptors responsible for the pancreatic secretion of insulin is dependent on the thyroid state; beta-adrenergic actions are predominant over alpha-actions in hyperthyroidism and vice versa in hypothyroidism.

Carbohydrate metabolism in hypothyroid myopathy

The carbohydrate metabolism in hypothyroid patients was investigated. After an overnight fast, the blood glucose level was 24% lower and the blood lactate level was 35% lower in the untreated hypothyroid patients than that observed in the treated hypothyroid patients or in the normal subjects. There was no difference in the blood alanine or plasma free fatty acid values between the subject groups. Skeletal muscle biopsied from two hypothyroid patients with marked myopathy showed normal glycogen content, 0.83%-0.86% (normal 1.06%), but reduced activity of acid maltase, 32-50 nmoles/min/g (normal 97). Forearm ischemic stimulation applied to hypothyroid patients failed to elevate the level of lactate. The results are compatible with impaired glycogenolysis from the skeletal muscle, which may be a contributory factor in the myopathy in hypothyroidism.

The action of thyroid hormone

Thyroid hormone affects both developmental and metabolic processes. It has a relatively specific effect on the synthesis of a number of enzymes and other proteins. The fundamental cellular mechanism of action seems to be at the level of genetic regulation. It involves interaction with nuclear receptors, leading to an activation of the protein synthesizing machinery. How binding to receptors is coupled to genetic activation is completely unknown. At least part of the metabolic effects of thyroid hormone could be mediated through an interaction with mitochondria and cell membrane, and with some enzymatic systems such as adenylcyclase.

Effect of thyroid hormones on microsomal fatty acid chain elongation synthesis in rat liver

Evidence is presented that rat liver microsomal fatty acid chain elongation synthesis and desaturation, as well as acetyl-CoA carboxylase and fatty acid synthetase, are strongly influenced by thyroid hormone level. Conversely, the fatty acid chain elongation system in mitochondria, unlike the oxidative capacity of palmitate, NADH, succinate and malate, does not seem significantly affected by the thyrotoxic state. In triiodothyronine-induced or thyroxine-induced hyperthyroidism, rat liver acetyl-CoA carboxylase, fatty acid synthetase and microsomal chain elongation and desaturation reactions are not greatly affected after the first 10 days of treatment, while after longer intervals a respective increase in these activities is shown of up to 87, 116 and 65% after 22 days. In propylthiouracil-induced hypothyroidism, all the above synthetic activities are strongly reduced immediately after three days of drug administration and diminished no further following longer periods. Although the pattern of synthesized fatty acids in the thyrotoxic state is similar to that obtained from normal subcellular rat fractions, the esterification process of fatty acids in microsomal lipids appears to be slightly inhibited in hypothyroid rats and increased following triiodothyronine or thyroxine administration. Finally, a reduction in the hepatic cyclic AMP level of about 41% is reported after 19 days of triiodothyronine-administration to rats. On the basis of the observed insensitivity of the mitochondrial fatty acid chain elongation system to the thyrotoxic state, a tentative interpretation of its role in the hepatic cell is postulated.

The effect of glucose, insulin and noradrenaline on lipolysis, and on the concentrations of adenosine 3':5'-cyclic monophosphate and adenosine 5'-triphosphate in adipose tissue

Glycerol release and tissue concentrations of ATP and cyclic AMP were followed during the incubation of adipose tissue with or without glucose, insulin and noradrenaline. Glucose plus insulin or, to a lesser extent, glucose alone increased the accumulation of glycerol during incubations both with and without noradrenaline by slowing the decline in the rate of glycerol release with time. Insulin alone decreased the accumulation by accelerating the fall in glycerol release. In the absence of noradrenaline, ATP and cyclic AMP concentrations were not significantly affected by insulin or glucose. With noradrenaline or noradrenaline plus insulin the ATP concentration gradually fell. With noradrenaline plus glucose the ATP concentration fell rapidly and then stabilized, or, if insulin was also present, returned to the control value. In the presence of noradrenaline, the concentration of cyclic AMP rose during the first 20min and then fell. Insulin lowered the peak concentration of cyclic AMP, but glucose had no effect either on the peak value or the fall in the concentration of the nucleotide. The increase and fall in the concentration of cyclic AMP with noradrenaline or noradrenaline plus insulin bore similarities to the increase and decline in the lipolytic rate in incubations without glucose. It is proposed that glucose stimulates ATP production by furnishing glycerol 1-phosphate and thus removing free fatty acids, but that it can influence lipolysis by a mechanism which is distinct from any which is mediated by free fatty acids, possibly by inhibiting the inactivation of the lipase.

The effect of noradrenaline on glyceride synthesis and oxidative metabolism in vitro in the brown fat of newborn rabbits

1. The effect of noradrenaline on the synthesis of glyceride from [U-(14)C]glucose and on gas exchange in the brown fat of newborn rabbits in vitro was investigated. 2. The specific radioactivity of l-glycerol 3-phosphate was lower than that of lactate, presumably because glycerol derived from glyceride was rephosphorylated by glycerokinase. 3. In the basal state more than 25% of the total respiration was due to pyruvate oxidation. Noradrenaline stimulated glyceride synthesis and total respiration without changing the proportion of the total respiration due to pyruvate oxidation. 4. The extra ADP released by noradrenaline stimulation of glyceride synthesis could not have supported more than 2% of the observed increase in substrate oxidation if mitochondria from brown-fat-cells remain fully coupled in the stimulated state, but could have supported about one-third of the observed increase if they become uncoupled in the presence of noradrenaline.

Biochemistry of hyperthyroidism and hypothyroidism

The thyroid hormones act directly on mitochondria, and thereby control the transformation of the energy derived from oxidations into a form utilizable by the cell. Through their direct actions on mitochondria, the hormones also control indirectly the rate of protein synthesis and thereby the amount of oxidative apparatus in the cell. A rationale for the effects of thyroid hormone excess or deficiency is based upon studies of the mechanism of thyroid hormone action. In hypothyroidism, slow fuel consumption leads to a low output of utilizable energy. In hyperthyroidism, rapid fuel consumption leads to a high energy output, but as efficiency decreases, the utilizable energy produced decreases. Many of the chemical and physical features of these diseases can be reduced to changes in available energy.