Causes of Low FSH and LH

Primary Mechanism

Low FSH and LH levels result from secondary (hypogonadotropic) hypogonadism, which is caused by impairment of the hypothalamic-pituitary-gonadal (HPG) axis rather than primary gonadal failure, according to the European Association of Urology 1
The fundamental pathophysiology involves reduced pulsatile secretion of GnRH from the hypothalamus, which leads to decreased LH pulse frequency and reduced FSH secretion, as reported by the European Society of Human Reproduction and Embryology 2, 3

Kallmann syndrome (IHH with anosmia) is a congenital cause of low FSH and LH, as stated by the European Association of Urology 1
Normosmic IHH is another congenital cause of low FSH and LH, according to the European Association of Urology 1

Testosterone or androgenic anabolic steroids suppress the HPG axis through negative feedback, leading to low FSH and LH, as reported by the European Association of Urology 1
Opiates are a common and often overlooked cause of low FSH and LH, according to the European Association of Urology 1

Type 2 diabetes mellitus/metabolic syndrome is associated with low FSH and LH, as stated by the European Association of Urology 1, 4, 6
Obesity affects the HPG axis, leading to low FSH and LH, according to the European Society of Human Reproduction and Embryology 5
Cushing syndrome (glucocorticoid excess) is a systemic disease that can cause low FSH and LH, as reported by the European Association of Urology 1, 4, 6

In secondary hypogonadism, FSH and LH are low or inappropriately normal despite low sex steroids, whereas primary gonadal failure causes elevated FSH and LH, as stated by the European Association of Urology 1
The LH/FSH ratio may be <2 in secondary hypogonadism, compared to >2 in some conditions like PCOS, according to the European Neurology Society 5