Effects of an acute water load on plasma ANP and AVP, and renal water handling in hypothyroidism: comparison of before and after L-thyroxine treatment.

To assess whether atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) participate in impaired water excretion in patients with hypothyroid states (HS), an oral acute water loading (WL) test (20 ml/kg.BW/45 min) was performed before and after L-thyroxine (T4) treatment in 5 hypothyroid patients. Plasma ANP, AVP, osmolality (Posm), total protein and renal water excretion were simultaneously determined, and these data were compared to the data from five normal subjects (NS). The impaired water excretion rate in HS was entirely improved in the euthyroid states (ES) after T4 therapy for at least 7 months. Plasma ANP in HS was lower than that in NS (5.9 +/- 0.9 vs. 16.5 +/- 3.6 pmol/L, P < 0.05), but increased after T4 treatment (21.2 +/- 5.7 pmol/L, P < 0.05). Plasma AVP in HS (1.6 +/- 0.5 pmol/L) showed a tendency to be lower than those in ES and NS (2.9 +/- 0.4 and 2.9 +/- 0.7 pmol/L), but did not respond to a fall in Posm after WL, unlike ES and NS. Significant positive correlations were noticed between Posm and plasma AVP in ES and NS, but not in HS. These results suggest that not only the impaired release and/or metabolisms of AVP and ANP, but also derangement of renal water and electrolytes handling might induce attenuation of CH2O formation in hypothyroid states.