An oestrogen-producing seminoma responsible for gynaecomastia.

In feminising testicular tumours, oestrogens can be either secreted by the tumour itself or produced by normal Leydig cells in response to paracrine and/or endocrine stimulation by hCG. Typical hormonal Leydig cell tumour patterns include: plasma oestradiol levels > 300 pmol/l on day 3 following an hCG injection, reduced plasma testosterone, and normal plasma hCG and gonadotrophin levels. Except for elevated plasma oestradiol levels, opposite results are observed in seminomas.