Possible stimulation of thermogenesis in brown adipose tissue by thyroid-stimulating hormone.

It is suggested that in addition to stimulating the thyroid gland (i.e., the main regulator of metabolic-rate in adults) thyroid-stimulating hormone (T.S.H.) stimulates the second thermoregulatory organ (i.e., the brown adipose tissue). Brown fat functions as a thermogenic organ in hibernating animals, in newborn infants, and during cold acclimatisation. However, B.F. may persist in childhood and in some adults. Its hypertrophy in response to T.S.H. could account for certain unexplained features of myxoedema in which serum-T.S.H. is raised, such as swelling of the supraclavicular fat pad and the less commonly encountered symptoms of ascites or pericardial and pleural serous effusions which can persist for years in undiagnosed cases and respond rapidly to thyroxine when serum-T.S.H. returns to normal. Lack of thyroxine is not the cause of these features since they are not found in pituitary myxoedema, where thyroid hormone levels are as low but T.S.H. is absent.