At first glance the physical symptoms of Kallmann syndrome and Klinefelter syndrome look very similar. Both can produce men with above average height but with under developed secondary sexual characteristics with small testicular volume, low testosterone levels (hypogonadism) and normally infertile.

However the causes of the low testosterone and infertility are almost the total opposite.

In Klinefelter syndrome there is an over production of gonadotrophins (LH and FSH) from the pituitary gland. (known as a hypergonadotrophic condition)

In Kallmann syndrome there is an absence of gonadotrophins (LH and FSH) from the pituitary gland. (known as hypogonadotrophic condition)

In Klinefelter syndrome there is a primary failure of the testes to function correctly (known as primary hypogonadism)

In Kallmann syndrome there is secondary failure of the testes to function. This is to say that they are in a dormant state because they have not had the correct gonadotrophin signals (LH and FSH) in order to function correctly. (known as secondary hypogonadism)

Klinefelter syndrome is caused by a precise genetic abnormality which is easily tested for, most normally an additional X chromosome. While a “normal” male has the genetic make up 46, XY a man with Klinefelter syndrome normally has the genetic make up of 47, XXY. It is the presence of this extra X chromosome which prevents the testes from functioning correctly, but the exact reason why this should be is not fully known.

The exact genetic cause of Kallmann syndrome is not fully understood and at least 6 different genes have been implicated as causes. There is no easy reliable genetic test for Kallmann syndrome; males will have the genetic make up 46, XY.