**5. Conclusions**

Diagnosis and consequently early treatment of the 17-HSD3 deficiency is frequently difficult because clinical signs are often mild or absent from birth until puberty. Moreover, the 17-HSD3 deficiency is clinically indistinguishable from other forms of 46,XY DSD such as AIS or 5-reductase 2 gene deficiency. The correct diagnosis can be arrived at by systematic endocrine evaluation and, most importantly, by the calculation of the T/4-A ratio. The diagnostic power of biochemical parameters is not always specific, because no normal reference range has yet been established in strictly age-matched controls and because of overlapping with other causes of 46,XY DSD due to impaired T biosynthesis. Molecular genetic testing confirms the diagnosis and provides the orientation for genetic counseling. A high index of suspicion should be present for any female who presents with inguinal hernias or mild clitoromegaly in infancy or early childhood. The virilization in the adolescent girl should also arouse suspicion. Since there are unique clinical implications based on the diagnosis of this condition, it is important to be as prompt and accurate as possible. In conclusion, endocrine evaluation is an important tool for the selection of patients with a suspected 17-HSD3 deficiency. In these patients, mutational analysis of the *HSD17B3* gene, supported by a knowledge of the ethnic distribution of mutations, is irreplaceable in confirming the diagnosis.
