Pericardial effusion has recently been reported as a complication of anorexia 
nervosa. A distinct pathophysiological cause of it could not be revealed. In 
some reports, there was a probable correlation between weight gain and reduction 
of pericardial effusion in anorexia nervosa cases. We encountered a case in 
which pericardial effusion remitted completely along with body weight increase 
and normalization of low T3 syndrome. These findings suggest that the reduction 
of pericardial effusion may correlate with both weight gain and low T3 
normalization. Plasma brain natriuretic peptide (BNP) levels were increased in 
this case despite heart failure, and plasma BNP decreased as pericardial 
effusion remitted. The measurement of serum BNP level may be a clinical 
parameter in such a case of pericardial effusion.