Causes of Low FSH Levels

Primary Pathophysiologic Mechanism

The American College of Endocrinology notes that reduced pulsatile secretion of GnRH from the hypothalamus disrupts the normal feedback loop between the gonads and the pituitary gland, distinguishing secondary hypogonadism from primary gonadal failure 1

Kallmann syndrome, a congenital failure of GnRH neuron migration, results in lifelong low FSH and LH, as reported by the Endocrine Society 1
Normosmic idiopathic hypogonadotropic hypogonadism causes low gonadotropins without olfactory defects, according to the European Society of Human Reproduction and Embryology 1
The American Urological Association notes that in males with azoospermia and FSH <7.6 IU/L, obstructive causes should be suspected rather than primary testicular failure 2

The American College of Clinical Endocrinologists states that testosterone or androgenic anabolic steroids suppress the HPG axis, causing profound suppression of both FSH and LH 1
Opiates are a frequently overlooked but common cause of hypogonadotropic hypogonadism with low FSH and LH, as reported by the American Society of Addiction Medicine 1
The European Society of Endocrinology notes that GnRH analogs, corticosteroids, and certain antiepileptic drugs can suppress gonadotropin secretion 3
The American College of Obstetricians and Gynecologists states that combined oral contraceptives completely suppress FSH and LH levels in women 4

Functional hypothalamic amenorrhea is characterized by both LH and FSH levels <2 IU/L, typically with LH <3 IU/L, caused by hypothalamic inhibition from stress, excessive exercise, eating disorders, or low body weight, as reported by the American Society for Reproductive Medicine 5, 4, 3
Energy deficit states, including eating disorders, excessive exercise, and preoccupation with weight, lead to hypothalamic suppression, according to the Academy of Nutrition and Dietetics 5, 4
Low BMI is strongly associated with lower FSH levels in women with functional hypothalamic amenorrhea, as noted by the American College of Sports Medicine 5

Type 2 diabetes mellitus and metabolic syndrome disrupt the HPG axis, leading to inappropriately low FSH and LH despite low sex steroid levels, as reported by the American Diabetes Association 1
Obesity affects gonadotropin secretion through multiple mechanisms, including increased aromatization of androgens to estrogens and altered GnRH pulsatility, according to the Obesity Society 1, 3
Cushing syndrome suppresses the hypothalamic-pituitary axis, resulting in low FSH and LH, as noted by the Endocrine Society 1
Chronic liver disease disrupts the hypothalamic-pituitary axis with low FSH and LH, leading to anovulation and amenorrhea in more than 25% of women with advanced disease, as reported by the American Association for the Study of Liver Diseases 3

Hyperprolactinemia suppresses GnRH pulsatility, leading to low LH and FSH, as noted by the Pituitary Society 3
Pituitary adenomas can cause central hypogonadism through mass effect or hormonal disruption, according to the American Association of Neurological Surgeons 3

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 reported by the American College of Endocrinology 1
The LH/FSH ratio may be <2 in secondary hypogonadism, compared to >2 in conditions like PCOS, as noted by the Endocrine Society 1
In males with azoospermia, FSH <7.6 IU/L suggests obstructive azoospermia rather than spermatogenic failure, particularly when testes are normal size with dilated epididymides, as reported by the American Urological Association 2