To explore a possible downward setting of the hypothalamic osmoreceptors in the hyponatremia of hypothyroidism, 4 adolescent patients with hypothyroidism were studied. Plasma and urine osmolality were measured on random paired simultaneous samples, and following a water load. The osmotic threshold was determined by the isovolemic infusion of hypertonic NaCl, and compared to the osmotic threshold of 6 healthy subjects. Random paired urine and plasma osmolality revealed inappropriately high urine osmolality for the given plasma osmolality. A water load produced a normal dilution of urine. Osmotic threshold was found at plasma osmolality of 286–287 mosm/kg, compared to 286.7 ± 1.0 in 6 normal individuals. It is concluded that none of the known types of the syndrome of inappropriate ADH secretion could account for the hyponatremia in the hypothyroid patients, and that patients with hypothyroidism have normal osmoreceptors as measured by the osmotic threshold test.