Water metabolism disturbances at different stages of primary thyroid failure

The aim of the present study was to study salt and water metabolism in thyr oid deficiency. We performed an oral water loading test (OWL) and a hypertonic 5% saline in fusion test (HSI) in 16 patients with overt primary hypothyroidism before r eplacement treatment (PRE group) and after, in eight patients with subclini cal hypothyroidism (SUB group) and in 16 normal individuals (CG group). In the PRE group, a lower free water clearance was detected in the OWL (P<0 .022), with lower plasma osmolality (OWL: P<0.005; HSI: P<0.001) and argini ne vasopressin (AVP) (OWL: P<0.001; HSI: P<0.001) than the CG group, across both tests; they normalized with the replacement treatment. The same plasm a abnormalities were detected in the SUB group with the HSI. Although the A VP and thirst thresholds did not differ between the groups, the lag between them was lower in the PRE (4.1 <plus/minus> 3.2 mOsm/kg) and SUB group (2. 6 +/- 2.1 mOsm/kg)than in the CG group (13.3 +/- 9.2 mOsm/kg) (P<0.05). The re were no differences in atrial natriuretic hormone (ANH), plasma renin ac tivity (PRA) and plasma aldosterone among the groups. These results indicate that plasma hypo-osmolality and low levels of AVP ar e present in primary hypothyroidism, and indeed are already present in the subclinical phase of the disease. An overlap between the thresholds of thir st and AVP seem to play a role in these abnormalities, but ANH, PRA and pla sma aldosterone do not appear to contribute.