AQP1 and hypothyroidism: In hypothyroidism, there is a fall in sodium and bicarbonate tubular reabsorption and impaired regulation of the expression of aquaporin 1 and 2 channels that are responsible for free water clearance in the distal tubules.3,6,18 The result is a loss of medullary hypertonicity and the ability to concentrate urine.19 There is a reversible increase in the sensitivity to ADH (anti diuretic hormone) at the collecting ducts that increases free water absorption.