The genetics of Kallmann syndrome and other forms of HH is a long way from being fully understood.

In a paper published in by the Proceedings of the National Academy of Sciences of the United States of America (PNAS) in 2010 a review listed 12 different genes that have been known, either in isolation or together that have been implicated in causing cases of KS or HH. Even with this number of genes involved over 60% of cases of KS and HH have an unknown genetic cause.

This means that there is no clear genetic test for KS or HH. Even if the you test for the two or three most common gene defects a negative result would not rule out a possible case of KS or HH.

PNAS 2010 Aug 107(34) 15140-4, Fig. 1

While Kallmann syndrome and HH cause a failure to enter puberty the root cause is located deep within the brain, specifically the communication between the pituitary and hypothalamus glands. It is a failure of the communication between these two glands that prevent the correct hormones being released by the pituitary gland which normally allow the development and correct function of the secondary sexual organs (testes / ovaries).

All the genes implicated so far in cases of KS / HH have some sort of role in the development of the communication pathways between the pituitary and hypothalamus. The pituitary and hypothalamus glands control many functions around the body. KS and HH are very specific conditions where only the sex hormone production side of their function is normally affected, leaving all the other functions intact. This is partly the reason why it is very a very difficult condition to detect early.

Apart from the lack of sense of smell, which is not unique, there is normally nothing to suggest a person has the condition until puberty fails to start. This makes diagnosis quite difficult sometimes, especially if doctors are not familiar with the condition. It is not unknown for some people with KS or HH to "know" they have a problem when young, but find it impossible to prove it or to persuade doctors to take them seriously.

Without a reliable genetic test too many people with KS or HH will be told they are "late bloomers" or "late starters" well into their late teens and are prevented from getting an early diagnosis or treatment.