FSH vs LH: What Each Hormone Does, How the Two Tests Differ, and How to Read Them Together

The pituitary gland sits at the base of the brain and steers the ovaries and testes with two hormones. FSH tells the ovary to grow a follicle each cycle (and tells the testes to make sperm). LH delivers the surge that makes that follicle release its egg, then keeps the resulting corpus luteum producing progesterone; in men, LH tells the Leydig cells of the testes to make testosterone. When the ovaries or testes underperform, the pituitary pushes harder and both hormones rise. When the problem is upstream — in the pituitary or hypothalamus — both fall. And in a few conditions the two diverge, which is a clue in itself.

FSH is reported in mIU/mL. In women it is low in the early cycle, rises modestly as a follicle is selected, spikes briefly with LH at ovulation, and falls in the luteal phase. Its most-used reading is the day 2–4 baseline: a rising early-cycle FSH means the ovary needs more stimulation to produce a follicle, which is the classic marker of declining ovarian reserve. After menopause FSH stays persistently high — often above 25–40 mIU/mL — because there are no follicles left to respond.

In men FSH supports sperm production, so it is measured in infertility evaluations: a high FSH with a low sperm count points to a problem in the testes themselves. For ovarian-reserve questions in women, guidelines note that AMH is a steadier marker than FSH because it does not swing with the cycle; the two are often ordered together. The FSH reference page lists the phase-specific ranges.

LH is also reported in mIU/mL and moves more dramatically than FSH. In women it sits low for most of the cycle and then surges to several times its baseline for about a day, roughly 24–36 hours before ovulation — the surge that ovulation predictor kits detect in urine. In the luteal phase it supports progesterone production. After menopause it is persistently high, like FSH.

LH's special value is in two patterns. In polycystic ovary syndrome, LH often runs high relative to FSH (a ratio above about 2:1 is a common, though not diagnostic, finding), which supports the diagnosis alongside irregular cycles, androgen excess and ultrasound findings. In men, LH is the number that says where low testosterone is coming from: high LH with low testosterone means the testes are not responding (primary hypogonadism); low or normal LH with low testosterone means the pituitary is not sending the signal (secondary hypogonadism), which changes the work-up entirely. The LH reference page has the ranges.

Almost always both, from the same draw — the cost difference is small and the pattern is the point. The question is really what else to add:

• Irregular or absent periods: FSH and LH with estradiol, prolactin and TSH — the standard first set, which separates PCOS, ovarian insufficiency, pituitary causes and thyroid disease. The Women's Hormone Panel covers it.
• Fertility planning or trying to conceive: day 2–4 FSH, LH and estradiol, plus AMH for reserve; mid-luteal progesterone if you want to confirm ovulation. The Women's Fertility Essentials panel is built around this.
• Perimenopause or menopause questions: FSH with estradiol. A persistently high FSH with low estradiol fits menopause, but in women over 45 guidelines diagnose on symptoms and age; a single value during perimenopause can be misleading because levels swing.
• Low testosterone in men: FSH and LH alongside total and (calculated) free testosterone, drawn in the morning — the LH result decides whether the problem is the testes or the pituitary. The Men's Hormone Panel pairs them.
• Male infertility: FSH, LH and testosterone with a semen analysis arranged separately.

Neither hormone requires fasting. Cycle day is everything for women — note it on the report, and never compare a mid-cycle LH surge with a day-3 baseline. Hormonal contraception suppresses both, so results while on it reflect the contraceptive. For men, draw in the morning with testosterone.

The FSH/LH pattern narrows the cause; the next tests confirm it. High-high in a woman under 40 is repeated and, if persistent, evaluated for primary ovarian insufficiency, which has implications for bone and heart health beyond fertility. The PCOS pattern is completed with androgens, glucose and lipid testing, since PCOS carries metabolic risk. Low-low is worked up with prolactin, TSH and a history of weight, training and stress; a pituitary MRI follows if prolactin is high or other pituitary hormones are off. In men, high LH prompts a testicular evaluation and often a karyotype; low LH prompts prolactin, iron studies and a medication review before any testosterone therapy is considered — because treating secondary hypogonadism with testosterone without finding the cause can miss a pituitary problem and suppresses fertility.

FSH and LH are routine draws at Quest and Labcorp, orderable through TestWell with physician review, results in a few business days with cycle-phase reference ranges on the report.