Hypothalamic disorders cause reduced TSH secretion by impairing the production or transport of TRH to the pituitary gland. Hypothyroidism may occur because the pituitary secretes TSH in insufficient quantities, or secretes TSH with an abnormal glycosylation pattern which reduces the biologic activity of TSH 1,2,3 . Treatment with oral TRH restores the biologic activity of TSH, suggesting that deficient hypothalamic TRH release induces both quantitative and qualitative abnormalities of TSH secretion. TSH molecules with reduced biologic activity may retain their immunologic reactivity in TSH immunoassays, explaining the sometimes observed slightly increased values of serum TSH (up to 10 mU/l) in central hypothyroidism 18, 23 .
Renal parenchymal disease can be a cause or consequence of hypertension. Progressive renal damage is caused by the mechanical and humoral effects of glomerular hypertension. The renal damage decreases the kidneys' ability to excrete salt and excess fluid (resulting in a low renin state, as opposed to the high renin state found in renovascular hypertension), and the hypertension worsens. As renal damage progresses, hyperparathyroidism develops and erythropoietin production increases, exacerbating the hypertension. 5 , 18 Thus, a vicious cycle of worsening renal function and hypertension begins.