What do FSH levels indicate about a person's health?

What FSH Levels Indicate About Health

FSH (follicle-stimulating hormone) is a pituitary hormone that primarily reflects reproductive function and ovarian/testicular reserve, with elevated levels indicating declining gonadal function in both sexes, and measurement is essential for diagnosing hypogonadism, premature ovarian insufficiency, and assessing fertility status.

Understanding FSH Physiology

FSH is a glycoprotein hormone synthesized in the anterior pituitary gland that stimulates sex steroid production and gamete development 1, 2. The hormone operates through a negative feedback loop: when ovarian or testicular function declines, FSH levels rise in response to decreased inhibin, estradiol (in women), or testosterone (in men) 3, 4.

Clinical Applications of FSH Testing

In Women

Premature Ovarian Insufficiency (POI):

• FSH measurement is diagnostic when two elevated FSH levels in the menopausal range (typically >25-40 IU/L depending on assay) occur before age 40 with amenorrhea ≥4 months 5.
• Both hormonal criteria AND clinical amenorrhea are required; a single elevated FSH is insufficient for diagnosis 5.
• Repeat testing in 4 weeks is mandatory, including FSH, estradiol, and LH to confirm the diagnosis 5.

Menopausal Transition:

• FSH levels increase 10-15-fold after menopause, with low estradiol and undetectable inhibin 4.
• However, FSH measurement has limited diagnostic value during perimenopause because levels fluctuate dramatically—postmenopausal FSH levels may be followed by normal ovulatory cycles 4.
• FSH can rise into the postmenopausal range and fall again into fertile ranges during the menopausal transition 4.

Fertility Assessment:

• FSH testing evaluates ovarian reserve in women with irregular menses, primary or secondary amenorrhea, or clinical signs of estrogen deficiency 6.
• Baseline FSH, LH, and estradiol should be obtained at age 13 in patients with delayed puberty or as clinically indicated 6.

In Men

Hypogonadism Diagnosis:

• Morning total testosterone should be measured first in men with symptoms (fatigue, weight loss, loss of libido, erectile dysfunction, depressive symptoms, or reduced bone density) 6.
• Once low testosterone is confirmed, measuring LH and FSH determines whether hypogonadism is primary (testicular failure with elevated FSH/LH) or central (hypothalamic/pituitary dysfunction with low or normal FSH/LH) 6.
• In men with chronic liver disease, a free testosterone index (total testosterone/SHBG ratio) <0.3 indicates hypogonadism, and FSH/LH help differentiate the cause 6.

Fertility Evaluation:

• FSH testing is indicated in men exposed to gonadotoxic treatments (alkylating chemotherapy, testicular radiation >2 Gy) to assess testicular function 6.
• Baseline FSH, LH, and testosterone should be obtained at age 14 in patients with delayed puberty or clinical signs of testosterone deficiency 6.

Key Diagnostic Patterns

Elevated FSH Indicates:

• Primary gonadal failure (ovarian or testicular dysfunction) when combined with low sex steroids 6, 5.
• Menopause or POI in women with amenorrhea 5, 4.
• Impaired follicular development in women, as FSH rises when inhibin production from developing follicles declines 3, 4.

Low or Normal FSH with Low Sex Steroids Indicates:

• Central (hypothalamic/pituitary) hypogonadism requiring further evaluation for pituitary pathology 6.

Critical Caveats

FSH measurement has significant limitations:

• During perimenopause, FSH fluctuates wildly and cannot reliably predict fertility or menopausal status 4.
• A single elevated FSH does not confirm POI; repeat testing with amenorrhea ≥4 months is mandatory 5.
• FSH levels vary throughout the menstrual cycle, being highest in early follicular phase 4.
• In men, morning samples are preferred due to diurnal testosterone variation, though FSH itself is less affected 6.

Secondary causes must be excluded:

• Thyroid dysfunction, hyperprolactinemia, and other endocrine disorders can affect FSH levels 5.
• Medications and systemic illness can alter FSH secretion 5.

When to Measure FSH

Measure FSH when:

• Evaluating amenorrhea (primary or secondary) in women <40 years 5.
• Assessing male hypogonadism after confirming low testosterone 6.
• Monitoring patients after gonadotoxic cancer treatments 6.
• Evaluating delayed puberty in adolescents 6.
• Investigating infertility in either sex 6.

Do NOT rely on FSH alone when:

• Assessing perimenopausal symptoms—clinical assessment is more reliable 4.
• Making fertility decisions during menopausal transition—ovulation can occur despite elevated FSH 4.
• Diagnosing POI without confirming prolonged amenorrhea 5.