Effect of obesity and starvation on thyroid hormone, growth hormone, and cortisol secretion

Obesity and starvation have opposing effects on normal physiology and are associated with adaptive changes in hormone secretion. Although hypothyroidism is associated with some weight gain, less than 10% of obese people are hypothyroid (thyroid function is normal). Treatment of obesity with severely hypocaloric diets causes changes in thyroid function that resemble sick euthyroid syndrome. Untreated obesity also is associated with low growth hormone (GH) levels. Levels of insulin-like growth factor 1 (IGF-1), however, are normal. These changes are reversed by drastic weight reduction. Cortisol levels are abnormal in people with abdominal obesity who exhibit an increase in urinary free cortisol but normal or decreased serum cortisol and normal adrenocorticotropic hormone (ACTH) levels. This increase in cortisol secretion may contribute to metabolic syndrome (insulin resistance, glucose intolerance, dyslipidemia, and hypertension). States of chronic starvation such as anorexia nervosa (AN) also are associated with changes in thyroid hormone, growth hormone, and cortisol secretion. Thyroid changes are consistent with findings in sick euthyroid syndrome. There is an increase in GH secretion, with a decrease in IGF-1 levels, and the changes in cortisol secretion resemble depression with increased urinary free cortisol and serum cortisol levels, but without changes in ACTH levels. The endocrine changes observed in obesity and starvation may complicate the diagnosis of primary endocrine diseases. The increase in cortisol secretion in obesity needs to be distinguished from Cushing's syndrome; the decrease in thryoid hormone levels in AN needs to be distinguished from secondary hypothyroidism; and the increase in cortisol secretion observed in AN requires a differential diagnosis with primary depressive disorder.