Causes of Follicle Stimulating Hormone Deficiency
Congenital:
Sexual differentiation is normal. In men, phallic development may be subnormal, resulting in a micropenis. Pubertal development is diminished or even absent, depending on the degree of gonadotropin deficiency.

Kallmann syndrome:
This is characterized by hypogonadotropic hypogonadism and 1 or more nongonadal congenital abnormalities, including anosmia, red-green blindness, midline facial abnormalities (eg, cleft palate), urogenital tract abnormalities, and neurosensory hearing loss. Hypogonadism in this syndrome is a result of deficient hypothalamic secretion of GnRH. Most cases are sporadic, but familial cases also occur.

Craniopharyngiomas:
These tumors arise from remnants of the Rathke pouch, which is the diverticulum of the roof of the embryonic oral cavity that normally gives rise to the anterior pituitary. Craniopharyngiomas are congenital malformations present at birth and gradually grow over the years. Approximately 75% arise in the suprasellar region. The most common presentation is due to increased intracranial pressure, including headaches and visual-field defects.

Combined pituitary hormone deficiency:
This results from a rare mutation in the gene encoding a transcription factor (PROP1), which is necessary for the differentiation of a cell type that is a precursor to somatotroph, lactotroph, thyrotroph, and gonadotroph cells, thus resulting in deficiencies in prolactin, thyroid-stimulating hormone (TSH), growth hormone (GH), FSH, and LH.

Fertile eunuch syndrome:
This is thought to represent an incomplete form of GnRH deficiency in men, in which an isolated and partial LH deficiency is present with low testosterone and normal FSH levels, resulting in preservation of spermatogenesis.

Abnormal beta subunit of LH:
This is a rare mutation in the LH beta subunit gene.

Abnormal beta subunit of FSH:
This is a rare mutation in the gene for the beta subunit of FSH, resulting in a low FSH level. This condition is encountered only in women but has been studied in male mice in which the FSH beta subunit gene has been knocked out. These mice have oligospermia but are fertile.

Hypothalamic/pituitary surgery:
If sufficient normal tissue is excised inadvertently, symptomatic hypogonadism may ensue initially, followed by dysfunction of other pituitary cells.