Short-term fasting usually does not significantly change follicle-stimulating hormone (FSH) in healthy adults. However, prolonged or severe calorie restriction, significant weight loss, or very low body fat can reduce hypothalamic GnRH secretion, suppressing LH and FSH and causing menstrual disturbances or infertility. Evidence for intermittent fasting is mixed; see a clinician if you notice menstrual or reproductive changes.

No — fasting is not usually required for an FSH (follicle‑stimulating hormone) blood test. For fertility evaluation, samples are often taken on day 3 of the menstrual cycle; otherwise timing is less critical (and in postmenopausal women timing generally doesn’t matter). Tell your clinician about medications, hormone therapy, or recent procedures, and follow any specific lab instructions.

During pregnancy FSH is normally very low—commonly below 1–5 mIU/mL (often under 2–3 mIU/mL). Levels are suppressed by high estrogen and progesterone and vary by trimester and laboratory assay. FSH testing isn’t routinely performed in pregnancy; interpret values using the lab’s reference range and discuss any unexpected results with your healthcare provider.

The preferred time for an FSH test is early in the menstrual cycle — cycle day 2–4, with day 3 being ideal — and a morning blood sample is often recommended. No fasting is usually required. If cycles are irregular or you’re assessing menopause, testing can be done any time but results may need repeat testing or complementary hormone tests (estradiol/LH) for accurate interpretation.

FSH shows a diurnal rise, with highest concentrations in the early morning—typically sampled around 8–10 AM. Levels vary with menstrual cycle phase and age (notably higher after menopause), so morning blood draws are recommended for consistency. Diurnal variation is modest compared with cycle-related changes, but timing helps reduce variability when assessing reproductive or pituitary function.

During fasting, glucagon levels rise markedly. Glucagon, released by pancreatic alpha cells, stimulates glycogen breakdown and gluconeogenesis to maintain blood glucose. Insulin falls while counter-regulatory hormones such as cortisol and growth hormone also increase to preserve energy and mobilize substrates. The glucagon-to-insulin ratio therefore shifts toward glucagon dominance during prolonged fasting.