Normalization of cellular zinc levels in patients with Downs syndrome does not always correct low thymulin levels.

Retrospective analysis of five Down's syndrome (DS) patients who presented with recurrent infection revealed that all had initial low thymulin levels. Three patients had low cellular zinc levels that normalized after zinc replacement. Contrary to previous studies, thymulin levels were persistently low in four of five DS patients despite maintaining or achieving normal cellular zinc levels. A primary thymic epithelial defect may be responsible for the persistent thymulin deficiency in DS patients.