5 alpha-reductase deficiency: diagnosis via abnormal plasma levels of reduced testosterone derivatives.

A case of 5 alpha-reductase deficiency is described who presented clinically as primary amenorrhoea. Plasma levels of the two isomers resulting from the 5 alpha-reduction of testosterone provide a simple method for diagnosing this condition. An intermediate abnormality was found in other family members consistent with the heterozygous state.