The presence of thyrotropin receptors (TSHR) has been reported in some 
extrathyroidal tissues but its physio-pathological role still remains unclear. 
TSH (seems to) affects the human erythrocytes Na(+)/K(+)-ATPase in vitro, 
however receptors on erythrocytes have not yet been described. In this work the 
effect of recombinant human TSH (rhTSH) on sites number and activity of 
erythrocyte Na(+)/K(+)-ATPase was investigated in a group of thyroidectomized 
patient enrolled for rhTSH test. As detected by (3)H-ouabain binding, rhTSH 
administration induced a significant increase in the number of sites (p=0.005) 
and in the Kd (p=0.006) of Na(+)/K(+)-ATPase. rhTSH did not induce significant 
difference in Na(+)/K(+)-ATPase activity measured by (86)Rb uptake. (125)I-TSH 
binding studies and Western blotting data showed the existence of TSHR in the 
erythrocytes of healthy donors. In conclusion The TSH action on 
Na(+)/K(+)-ATPase of human erythrocytes can be explained by the presence of 
TSHR.