Continued glucose output after re-feeding contributes to glucose intolerance in hyperthyroidism

The long-term oral administration of thyroxine: effects on blood hematological and biochemical features in broiler breeder hens

Published data on the beneficial effect of short-term administration of thyroxine (T₄) in broiler breeder hens to reduce the ascites incidence in their progeny chicks raises the question as to what extent might the long-term maternal administration of T₄ affect the blood hematological and biochemical attributes in breeder hens. A total of 70 broiler breeder hens (47-wk-old) were randomly allotted to control or thyroxine treated (T₄) groups. Pure T₄ (0.3 mg/bird per day) was orally administered to T₄ birds for 14 successive weeks, whereas the control group received the drinking water only. Blood samples were obtained from the brachial vein prior to the initiation of the trial as well as weeks 50, 53, 55, 57, 59, and 61 of age. Body weight was decreased but egg production was not affected by T₄ treatment. Plasma concentration of T₄, but not triiodothyronine (T₃), was increased in T₄-treated hens (P < 0.05). The total number of leukocytes and erythrocytes were also higher in T₄ birds. A significant effect of time was observed for erythrocyte number and plasma cholesterol concentration (P < 0.05). The long-term administration of T₄ did not affect the concentrations of serum calcium and plasma total protein, albumin, globulin, cholesterol, triglyceride, high density lipoprotein, low density lipoprotein, very low density lipoprotein, alanine amino transferase, and aspartate amino transferase (P > 0.05). However, serum concentrations of phosphorus, glucose, and alkaline phosphatase were higher in T₄ hens as compared to their control counterparts. In spite of differences in circulatory concentrations of a number of traits between the experimental groups, the recorded values were within their reference ranges. Therefore, the administration of T₄ for an extended period of time had no apparent adverse effect on the clinical profile in subjected hens, which may practically support the implementation of this preventative treatment as an approach to decrease the ascites incidence; however, a lower incidence rate in the progeny chicks produced from hens receiving T₄ for long-term periods of time remains to be elucidated.

Pancreatic and Islet Development and Function: The Role of Thyroid Hormone

A gradually expanding body of literature suggests that Thyroid Hormone (TH) and Thyroid Hormone Receptors (TRs) play a contributing role in pancreatic and islet cell development, maturation, and function. Studies using a variety of model systems capable of exploiting species-specific developmental paradigms have revealed the contribution of TH to cellular differentiation, lineage decisions, and endocrine cell specification. Moreover, in vitro and in vivo evidence suggests that TH is involved in islet β cell proliferation and maturation; however, the signaling pathway(s) connected with this function of TH/TR are not well understood. The purpose of this review is to discuss the current literature that has defined the effects of TH and TRs on pancreatic and islet cell development and function, describe the impact of hyper- and hypothyroidism on whole body metabolism, and highlight future and potential applications of TH in novel therapeutic strategies for diabetes.

Thyroid hormone analogues for the treatment of metabolic disorders: new potential for unmet clinical needs?

OBJECTIVE

To provide a comprehensive review of the discovery and development of selective thyroid hormone receptor agonists and provide a discussion of their use in hyperlipidemia, obesity, and type 2 diabetes mellitus.

METHODS

Preclinical and clinical English language literature from 1930 to present was reviewed and thematically summarized.

RESULTS

Human trials have shown that thyroid hormone receptor β (TRβ) agonists effectively lower low-density lipoprotein, triglycerides, apolipoprotein B, and lipoprotein(a) levels. In preclinical studies, TRβ agonists enhance reverse cholesterol transport and decrease atherosclerosis in selected models. While animal data suggest these drugs may have additional utility to modulate weight and improve glucose homeostasis, human studies have not shown similar results.

CONCLUSION

TRβ agonists are a novel therapeutic class for lipid management. Their mechanism of action for lipid lowering is distinct from statin drugs, suggesting a strong possibility for synergistic effects with combined therapy. The long-term effects of these drugs on cardiovascular outcomes, however, are unknown. Recently, the development of the most promising agent in this class, eprotirome, was halted over toxicology concerns following long-term canine studies. Consequently, the future of contemporary TRβ agonists is unclear. The creation of a next generation of TRβ agonists that provide additional tissue specific effects or bind TRβ with even higher selectivity may lead to improved safety and efficacy and allow for their application to other metabolic disorders like obesity and type 2 diabetes mellitus.

Hyperthyroidism-associated insulin resistance is not mediated by adiponectin levels

To evaluate the relationship between circulating adiponectin and insulin sensitivity in patients with hyperthyroid Graves' disease, we studied 19 adult patients with this disease and 19 age- and sex-matched euthyroid controls. All hyperthyroid patients were treated with antithyroid drugs and were re-evaluated after thyroid function normalized. Before antithyroid treatment, the adiponectin plasma concentrations were not different comparing with those in control group. The adiponectin levels remained unchanged after treatment. The homeostasis model assessment of insulin resistance (HOMA-IR) in hyperthyroid group was higher before treatment than after treatment. There was no significant difference in serum glucose and insulin levels between hyperthyroid and control groups and in the hyperthyroid group before and after treatment. BMI-adjusted adiponectin levels were not different among three groups. On the other hand, BMI-adjusted insulin levels and HOMA-IR values were significantly decreased after management of hyperthyroidism. Pearson's correlation revealed that insulin and HOMA-IR values positively correlated with triiodothyronine (T3) and free thyroxine (FT4) levels. However, adiponectin did not correlate with T3, FT4, insulin, HOMA-IR and thyrotropin receptor autoantibody (TRAb) levels. In conclusion, insulin resistance associated with hyperthyroidism is not mediated by the levels of plasma adiponectin.

Insulin action in hyperthyroidism: a focus on muscle and adipose tissue

Hyperthyroidism leads to an enhanced demand for glucose, which is primarily provided by increased rates of hepatic glucose production due to increased gluconeogenesis (in the fasting state) and increased Cori cycle activity (in the late postprandial and fasting state). Adipose tissue lipolysis is increased in the fasting state, resulting in increased production of glycerol and nonesterified fatty acids. Under these conditions, increased glycerol generated by lipolysis and increased amino acids generated by proteolysis are used as substrates for gluconeogenesis. Increased nonesterified fatty acid levels are necessary to stimulate gluconeogenesis and provide substrate for oxidation in other tissues (such as muscle). In the postprandial period, insulin-stimulated glucose uptake by the skeletal muscle has been found to be normal or increased, mainly due to increased blood flow. Under hyperthyroid conditions, insulin-stimulated rates of glycogen synthesis in skeletal muscle are decreased, whereas there is a preferential increase in the rates of lactate formation vs. glucose oxidation leading to increased Cori cycle activity. In hyperthyroidism, the Cori cycle could be considered as a large substrate cycle; by maintaining a high flux through it, a dynamic buffer of glucose and lactate is provided, which can be used by other tissues as required. Moreover, lipolysis is rapidly suppressed to normal after the meal to facilitate the disposal of glucose by the insulin-resistant muscle. This ensures the preferential use of glucose when available and helps to preserve fat stores.

Change of visfatin, C-reactive protein concentrations, and insulin sensitivity in patients with hyperthyroidism

The present study was undertaken to evaluate the change of circulating visfatin, C-reactive protein (CRP) concentrations, and insulin sensitivity in patients with hyperthyroidism. We studied 19 adult patients (14 women and 5 men aged 32.6 +/- 1.8 years) with hyperthyroidism due to Graves disease and 19 age- and sex-matched euthyroid controls (17 women and 2 men aged 36.7 +/- 2.7 years). All hyperthyroid patients were treated with 1 of 2 antithyroid drugs and were reevaluated after thyroid function normalized. Before antithyroid treatment, the hyperthyroid group had significantly higher visfatin plasma concentration (mean +/- standard error of the mean, 20.7 +/- 1.8 ng/mL) than the control group (16.2 +/- 1.3 ng/mL, P = .044); but the visfatin level dropped significantly after treatment (12.0 +/- 1.4 ng/mL, P < .001). The reciprocal index of homeostasis model assessment of insulin resistance (HOMA-IR) in the hyperthyroid group was higher before treatment (2.06 +/- 0.26 mmol mU/L*L) than after treatment (1.21 +/- 0.16 mmol mU/L*L, P = .027). There was no significant difference in serum glucose, high-sensitivity CRP, and insulin levels between hyperthyroid and control groups and in the hyperthyroid group before and after treatment. Body mass index-adjusted visfatin levels were significantly elevated in the hyperthyroid group. Pearson correlation revealed that visfatin, glucose, insulin, and HOMA-IR values positively correlated with triiodothyronine and free thyroxine levels. However, visfatin did not correlate with insulin and HOMA-IR levels. The results indicated that plasma visfatin concentration was elevated in hyperthyroidism due to Graves disease, but serum CRP levels were not. Plasma visfatin levels were not associated with indicators of insulin resistance in hyperthyroid patients.

Fuel selection and carbon flux during the starved-to-fed transition

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The disposition of carbohydrate between glycogenesis, lipogenesis and oxidation in liver during the starved-to-fed transition

A comparison was made between the time courses of restoration of pyruvate dehydrogenase activities, fructose 2,6-bisphosphate concentrations and lipogenic rates, together with net hepatic glucose flux and glycogen synthesis/deposition in livers of 48 h-starved rats provided with laboratory chow ad libitum for up to 24 h. Increased glycogenesis, lipogenesis and net glucose uptake were observed after 1 h of re-feeding, preceding re-activation of pyruvate dehydrogenase, which occurred after 3-4 h. Increased concentrations of fructose 2,6-bisphosphate were only observed after 5-6 h. The implication of the temporal relationship between these parameters is discussed.

Regulation of hepatic fructose 2,6-bisphosphate concentrations and lipogenesis after re-feeding in euthyroid and hyperthyroid rats. A regulatory role for glycogenesis

The time courses of restoration of fructose 2,6-bisphosphate (Fru-2,6-P2) concentrations and rates of lipogenesis after chow re-feeding were correlated with glycogen concentrations and rates of glycogen synthesis in livers of 48 h-starved euthyroid and hyperthyroid rats. Although a regulatory function for glycogen in the regulation of Fru-2,6-P2 concentrations was excluded, an inverse relationship between rates of glycogenesis and Fru-2,6-P2 concentrations indicated a role for glycogenesis in the suppression of Fru-2,6-P2 concentrations during the early (0-4 h) period of re-feeding. There was also a negative correlation between rates of glycogenesis and lipogenesis, and a positive correlation between glycogen concentrations and the lipogenic rate. Decreased rates of glycogenesis in hyperthyroid rats were associated with increased rates of lipogenesis. The response of Fru-2,6-P2 to changes in the glycogenic rate was modified by hyperthyroidism, although a negative correlation was again observed.