🩸 Test on specific cycle days
FSH, LH, Estradiol
Days 2–5 of your period (Day 1 = first day of full flow). These hormones fluctuate dramatically across the cycle — mid-cycle or luteal phase values are not comparable to follicular reference ranges.
Progesterone
Day 21 of a 28-day cycle, or 7 days after confirmed ovulation (use LH strip or BBT). Follicular-phase progesterone is normally near zero — a "low" result from Day 5 is meaningless.
AMH
Any day of the cycle — AMH is the one hormone marker that does not fluctuate significantly with the menstrual cycle.
LH surge
If testing to detect ovulation, LH peaks 24–36 hours before ovulation — typically around Day 12–14 of a 28-day cycle, but variable.
🚫 Fasting required (8–12 hours)
Fasting Insulin
Must be fasting — even a small snack raises insulin significantly, making the result useless for diagnosing insulin resistance. Water only for 8–12 hours beforehand.
Homocysteine
Recommended fasting (12 hours) for most accurate baseline. Food — particularly protein — transiently raises homocysteine.
HbA1c
Fasting not strictly required (reflects 3-month average) — but most labs draw it alongside a fasting glucose panel. Avoid if you've had a blood transfusion in the past 3 months.
🌅 Morning only (before 10am)
TSH & Free T4
TSH follows a diurnal rhythm — it is highest in early morning and falls through the day. Afternoon TSH values can be 30–50% lower than morning, potentially missing subclinical hypothyroidism. If on levothyroxine, take the test before your morning dose.
Prolactin
Draw fasting, after 30 minutes of seated rest, before any breast stimulation or strenuous exercise that day. Prolactin rises acutely with stress, eating, exercise, and orgasm — a single elevated result needs a calm, fasting retest to be meaningful.
DHEA-S
Ideally morning — DHEA-S follows the adrenal cortisol rhythm and is slightly higher earlier in the day.
✅ Any time, no special prep
Ferritin, B12, Folate
Not meal-dependent. Can be drawn any time of day. Note: ferritin is an acute-phase reactant — it rises with inflammation or infection even when iron stores are actually low. Don't test during illness.
Vitamin D (25-OH)
No special timing. Best tested in late winter or early spring for the most clinically meaningful reading (your annual low point). Avoid testing within 48 hours of a large bolus dose.
Hemoglobin, MCV
No special timing — typically part of a full blood count (FBC/CBC).
hs-CRP
Do not test during illness, injury, or immediately after intense exercise — all acutely raise CRP. Wait at least 2 weeks after recovering from any infection for a meaningful baseline reading.
If your cycle is irregular or absent: For PCOS, hypothalamic amenorrhoea, or perimenopause, cycle-day timing guidelines may not apply. In these cases, draw FSH, LH, and estradiol on any day and interpret in the context of your condition — or ask your doctor which day they want the blood drawn.