Erythrocyte Na + /K + -ATPase is increased in subjects with subclinical hypothyroidism

OBJECTIVE In erythrocytes of patients with overthyperthyroidism, the number of ouabain-binding sites and the activity of the Na+/K+-ATPase have been demonstrated to be decreased, whereas the opposite is true in patients with overt hypothyroidism. Noinformation has been reported on the status of the Na+/K+-ATPase in subclinically hypothyroid (Sub Hypo)patients. DESIGN We investigated the number of ouabain-bindingsites and Na+/K+-ATPase activity in erythrocytes ofchronic Sub Hypo subjects. PATIENTS AND METHODS We measured 3 H-ouabainbinding sites in erythrocytes from 15 patients with subclinical hypothyroidism, and compared with those found in 17 normal subjects (N), seven with overt hypothyroidism (Hypo) and 10 with overt hyperthyroidism (Hyper). The activity of the sodium pump was assessed by measuring ouabain-sensitive 86 Rb uptake in a subpopulationof the same groups. RESULTS The number of ouabain-binding sites in Sub Hypo patients (252 ±17; mean ± SEM) was significantly higher (P< 0·02) than in Hyper (135 ±12) and N (203 ±10) groups, whereas it was not significant different from Hypo (293 ±31). There was a positive correlation between the number of ouabain-binding sites and TSH concentrations (P< 0·002) when Sub Hypo and N groups were considered together. There was a negative correlation between the number of ouabainbinding sites and free thyroxine (FT4; P < 0·0001) and free triiodothyronine (FT3) concentrations (P < 0·001) when all subjects were considered. Ouabain-sensitive 86 Rb uptake (picomoles86Rb/h 10 6 cells) in Sub Hypo was significantly higher (4·2 ± 0·5) when compared with N (2·5 ± 0·2, P< 0·01) and Hyper (2·5 ± 0·5, P < 0·02). CONCLUSIONS Erythrocytes of subclinically hypothyroid patients show a significant increase in the number of ouabain-binding sites and in ouabain-sensitive 86 Rb uptake. The state of erythrocyte Na + /K+ -ATPase may therefore represent a biochemical marker of subclinical hypothyroidism.