Renal studies in the hypothyroid rat

Abstract

Whole kidney clearance studies were performed on 27 hypothyroid rats (H) and 25 euthyroid littermate rats (C). Absolute glomerular filtration rate (GFR) was less in hypothyroid rats (1.02 versus 1.39 ml/min), but when factored by body surface area (BSA) the two groups were comparable ( GFR BSA = 2.26 versus 2.24 μl /min/sq/cm ). Absolute and fractional (per GFR) sodium clearance was greater in the hypothyroid animals ( C Na GFR:H = 0.36, C = 0.11, p < 0.001 ), but total electrolyte (Na + + K +) excretion and clearance were comparable. End proximal tubular fluid (TF) micropuncture collections in 20 animals revealed no change in fractional reabsorption ( TF P inulin) in hypothyroid rats ( TF P insulin, 2.5 versus 2.7), but showed a decrease in single nephron GFR (Vo, 17 versus 36 nl/min) and absolute reabsorption (11 versus 20 nl/min). Hypothyroidism is associated with a decrease of sodium reabsorption at nephron sites distal to the convoluted proximal tubule. With ad libitum electrolyte and water intake most rats can maintain themselves in volume-sodium balance, but an occasional animal may display this distal leak syndrome in the extreme. One such rat had a urinary sodium concentration U Na of 147 mEq/liter and a urinary flow rate of 56.0 μl/min (H = 3.33, C = 1.39). In addition, its hydropenic normal end proximal TF P inulin mean of 2.7 confirmed the inability of the proximal nephron to compensate for this late leak. This rat's plasma sodium was 128 mEq/liter (H = 147, C = 148), dilutional hyponatremia. These renal problems are similar to those reported in man and suggest that the rat is an appropriate model for the study of human hypothyroidism.