Do luteinizing hormone (LH) and follicle‑stimulating hormone (FSH) blood tests require fasting?

LH and FSH Testing Does NOT Require Fasting

LH and FSH blood tests do not require fasting, though the timing during the menstrual cycle matters significantly for accurate interpretation in women. 1

Fasting Status for LH/FSH Testing

• No fasting is required for LH and FSH measurements according to established endocrine testing protocols 1
• These gonadotropins are measured as part of routine hormone panels without any dietary restrictions 2
• The lack of fasting requirement applies to both men and women across all age groups 1

Critical Timing Considerations (More Important Than Fasting)

For Women with Regular Menstrual Cycles

• Blood should be drawn between day 3 and day 6 of the menstrual cycle for baseline evaluation 2
• The measurement should be based on an average of three blood samples taken 20 minutes apart during this early follicular phase window for optimal accuracy 2
• This timing is essential because LH and FSH levels fluctuate dramatically throughout the menstrual cycle, with mid-cycle peaks occurring during ovulation 3, 4

For Men

• Morning collection is preferred when evaluating multiple hormone axes simultaneously, but this relates to circadian rhythm patterns rather than fasting status 1
• No specific time-of-day restriction is mandatory for isolated LH/FSH testing in men 5

Common Clinical Pitfalls to Avoid

• Do not confuse cycle timing with fasting requirements: The day 3-6 collection window is about capturing baseline gonadotropin levels during the early follicular phase, not about dietary preparation 2
• Avoid random-day testing in premenopausal women: LH/FSH ratios and absolute values vary dramatically across the menstrual cycle, making interpretation unreliable if drawn at the wrong time 2, 3
• Do not draw blood immediately post-seizure (postictal): While this applies more to prolactin, morning resting levels are recommended for comprehensive reproductive hormone panels 2

Abnormal Result Thresholds (For Context)

• In women being evaluated for reproductive endocrine disorders: LH/FSH ratio >2 suggests polycystic ovary syndrome, while FSH >35 IU/L indicates ovarian failure 2
• LH <7 IU/mL may indicate hypothalamic dysfunction 2
• These diagnostic thresholds are valid only when samples are collected at the appropriate time in the menstrual cycle 2

What Patients CAN Do Before Testing

• Eat and drink normally before LH and FSH blood draws 1
• Take regular medications unless specifically instructed otherwise by the ordering physician 2
• Women should track their menstrual cycle to ensure testing occurs during the correct window 2