Renal concentrating ability in obesity. Effect of modified fasting and the supplementation of T3, sodium chloride and carbohydrate

Mechanisms in endocrinology: the crosstalk between thyroid gland and adipose tissue: signal integration in health and disease

Obesity and thyroid diseases are common disorders in the general population and they frequently occur in single individuals. Alongside a chance association, a direct relationship between 'thyroid and obesity' has been hypothesized. Thyroid hormone is an important determinant of energy expenditure and contributes to appetite regulation, while hormones and cytokines from the adipose tissue act on the CNS to inform on the quantity of energy stores. A continuous interaction between the thyroid hormone and regulatory mechanisms localized in adipose tissue and brain is important for human body weight control and maintenance of optimal energy balance. Whether obesity has a pathogenic role in thyroid disease remains largely a matter of investigation. This review highlights the complexity in the identification of thyroid hormone deficiency in obese patients. Regardless of the importance of treating subclinical and overt hypothyroidism, at present there is no evidence to recommend pharmacological correction of the isolated hyperthyrotropinemia often encountered in obese patients. While thyroid hormones are not indicated as anti-obesity drugs, preclinical studies suggest that thyromimetic drugs, by targeting selected receptors, might be useful in the treatment of obesity and dyslipidemia.

Thyroid hormone therapy for obesity and nonthyroidal illnesses: a systematic review

CONTEXT

Thyroid hormone therapy to enhance weight loss in obesity during caloric deprivation and to improve morbidity and mortality in adults with nonthyroidal illnesses remains controversial.

OBJECTIVE

The aim of this study was to conduct a systematic review evaluating effectiveness and risks of T(3) and/or T(4) therapy in these populations.

DATA SOURCES

Electronic databases and reference lists were searched.

STUDY SELECTION

Studies with comparable control groups comparing T(3) and/or T(4) therapy to placebo in randomized controlled trials (RCTs) or prospective observational studies were selected.

DATA EXTRACTION

Three reviewers performed serial abstraction.

DATA SYNTHESIS

During caloric deprivation of obese subjects, T(3) therapy decreased serum TSH and T(4) concentrations. Consistent effects of T(3) or T(4) on weight loss, protein breakdown, metabolic rate, and heart rate could not be established. In euthyroid cardiac patients, T(3) decreased TSH and free T(4) levels, without consistent effects of T(3) or T(4) on heart rate, cardiac output, or systemic vascular resistance. Mortality increased 3.3-fold with T(4) therapy in acute renal failure patients, whereas an effect in cardiac, critically ill, and burn patients could not be established. Equivalence testing indicated that larger RCTs are required to determine whether thyroid hormone therapy alters end-points in obesity or nonthyroidal illnesses.

LIMITATIONS

Numbers of usable unique studies were small, numbers of patients in each study were inadequate, end-points were variable, few RCTs were performed, and study quality of non-RCTs was poor.

CONCLUSIONS

Available data are inconclusive regarding effectiveness of thyroid hormone therapy in treating obesity or nonthyroidal illnesses, whereas data support that such therapy induces subclinical hyperthyroidism.

24-h hydration status: parameters, epidemiology and recommendations

Hydration of individuals and groups is characterised by comparing actual urine osmolality (Uosm) with maximum Uosm. Data of actual, maximum and minimum Uosm in infants, children and adults and its major influencing factors are reviewed. There are remarkable ontogenetic, individual and cultural differences in Uosm. In the foetus and the breast-fed infant Uosm is much lower than plasma osmolality, whereas in children and adults it is usually much higher. Individuals and groups may show long-term differences in Uosm. In industrialised countries, the gender difference of Uosm is common. There are large intercultural differences of mean 24-h Uosm ranging from 860 mosm/kg in Germany, 649 mosm/kg in USA to 392 mosm/kg in Poland. A new physiologically based concept called 'free-water reserve' quantifies differences in 24-h euhydration. In 189 boys of the DONALD Study aged 4.0-6.9 y, median urine volume was 497 ml/24-h and median Uosm 809 mosm/kg. Considering mean-2 s.d. of actual maximum 24-h Uosm of 830 mosm/kg as upper level of euhydration and physiological criterion of adequate hydration in these boys, median free-water reserve was 11 ml/24-h. Based on median total water intake of 1310 ml/24-h and the third percentile of free-water volume of -156 ml/24-h, adequate total water intake was 1466 ml/24-h or 1.01 ml/kcal. Data of Uosm in 24-h urine samples and corresponding free-water reserve values of homogeneous groups of healthy subjects from all over the world might be useful parameters in epidemiology to investigate the health effects of different levels of 24-h euhydration.

The most essential nutrient: defining the adequate intake of water

Although water is quantitatively the most import nutrient, there are no recommended dietary allowances (RDA) or adequate intake (AI) values. Based on 718 assessments of 24-hour total water intake, urine volume, and urine osmolality, individual hydration status was characterized in 479 healthy boys and girls of the DONALD study aged 4.0 to 6.9 years and 7.0 to 10.9 years. Mean 24-hour total water intake ranged from 0.90 mL/kcal to 0.96 mL/kcal, and median 24-hour urine osmolality ranged from 683 mosm/kg to 854 mosm/kg. A maximum urine osmolality of 830 mosm/kg (mean - 2 SD) in healthy children with a typical affluent Western-type diet was the physiologic criterion of the upper limit of euhydration. "Water reserve" (24-hour urine volume - hypothetical urine volume to excrete 24 urine solutes at a concentration of 830 mosm/kg) was a quantitative measure of individual 24-hour hydration status and ensuring euhydration in 97% of the subjects in each group; AI values of total water in the 4 age and sex groups ranged from 1.01 mL/kcal to 1.05 mL/kcal. These procedures to quantify 24-hour hydration status may prove valuable in investigating the effects on health of different states of euhydration.