LH and FSH (Sex Hormones, Gonadotropins)

Understand

Overview

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are glycoprotein gonadotropins secreted by gonadotroph cells of the anterior pituitary gland. Together they form the main link between the central nervous system and the gonads — the ovaries in women and the testes in men — governing puberty, the menstrual cycle, sperm production, and the synthesis of sex hormones, estradiol and testosterone. The test measures both hormones' concentrations in blood serum, usually simultaneously, since their relationship to each other and to cycle phase or age carries far more information than a single value taken in isolation.

The clinical significance of this test lies in the fact that the LH and FSH pattern helps locate where along the hypothalamic-pituitary-gonadal axis a problem sits. High concentrations of both gonadotropins alongside low sex hormone levels point to gonadal failure itself (primary, hypergonadotropic hypogonadism) — the pituitary correctly detects the estradiol or testosterone deficiency and tries to compensate with increased LH and FSH secretion, but the gonads don't respond. The opposite pattern — low or inappropriately normal LH and FSH with low sex hormones — points to a problem higher up, in the hypothalamus or pituitary (secondary, hypogonadotropic hypogonadism), where the driving signal itself is insufficient.

The test is most often ordered for women with irregular or absent periods, in the infertility workup of both partners, when premature ovarian failure or menopause is suspected, and in assessing puberty — both delayed and precocious. In men, LH and FSH are measured mainly in the workup of hypogonadism, infertility, and abnormal semen parameters, where they help distinguish a testicular cause from a hypothalamic-pituitary one.

Interpreting the result in women is especially context-dependent, since both hormones' concentrations change repeatedly over a single menstrual cycle. FSH is relatively highest at the start of the follicular phase, gradually declines as the dominant follicle matures, then both hormones surge sharply twelve to thirty-six hours before ovulation — it's this sharp LH surge that directly triggers ovulation. After ovulation, in the luteal phase, both hormones' concentrations decline again. For this reason, a single LH or FSH result only makes sense alongside the cycle day it was drawn on, and a test for early ovarian reserve is usually ordered on cycle day two or three. In postmenopausal women, both hormones, especially FSH, are persistently high, since the ovaries stop responding and the negative feedback from estradiol disappears.

In men, LH and FSH concentrations are much more stable over time than in women, though they still show pulsatile secretion on an hourly scale, which is why several measurements at short intervals are sometimes ordered when a single result is ambiguous. It's also worth knowing that the popular LH-to-FSH ratio, historically used in diagnosing polycystic ovary syndrome, has, according to more recent research, limited diagnostic value on its own and shouldn't be the sole diagnostic criterion.

Logistically, the test usually doesn't require strict fasting, but in women it's crucial to note the menstrual cycle day at the time of the draw, and the use of hormonal contraception or other hormone therapy should be reported to the ordering physician, since it strongly suppresses natural gonadotropin secretion and makes the result uninterpretable in the standard way.

It's worth emphasizing that this test evaluates only the laboratory parameter itself and the hormonal axis's regulatory mechanism — it doesn't replace a full fertility assessment or infertility workup, which covers a much broader range of clinical, imaging, and partner-based tests. LH and FSH are one piece of that puzzle, not the whole workup.