While under hormone replacement therapy (HRT) you are taking medication containing the hormones that the testes / ovaries would normally be producing. In the majority of cases they are not going to produce fertility.

A very small number of people with Kallmann’s syndrome or HH do become fertile while on HRT, some in fact go through almost total normal puberty late in life. Just how this happens is still open for debate and only occurs in less than 10% of KS / HH cases.

In the majority of cases to achieve fertility you have to have a different form of treatment instead of HRT, namely the pituitary or hypothalamic hormones.