A case of adrenal insufficiency secondary to syphilis and difficulties in the diagnostic work-up.

We report a case of Addison's disease presumed to be secondary to syphilis. The patient presented with adrenal failure and a maculopapular rash on his trunk and palms. Syphilis was suspected but the non-treponemal serological test used (the rapid plasma reagin test (RPR)) was falsely negative due to the prozone phenomenon. Treatment with benzathine penicillin resulted in normalization of adrenal function and resolution of the cutaneous findings. Repeat testing of the previously obtained serum sample showed the RPR to be positive on increasing dilutions of serum, consistent with the prozone effect.