How to Prepare for an Iron and Hormone Blood Test
Follow your own lab's preparation rules. They beat everything on this page. The fork is simple: are you having only ferritin checked, the iron your body keeps in storage, or hormones as well? Ferritin alone is the steadiest test and needs the least preparation. Add sex hormones, thyroid or cortisol and the time of day, your cycle and what you take all start to move the result. Below: the exact question to ask the lab, what to tell the nurse, and what to write next to the number so it can be read properly later.
Ask the lab for its own preparation rules before anything else
Look at the request form or the booking message and write down which tests were ordered. Then phone the lab or the surgery and say: "Which of these tests need fasting or a set time of day?" Ask about water in the same call.
Write the answer down in the lab's own words. Fasting and timing rules differ between labs, so a rule that is right for one can be wrong for yours. A sample taken outside the lab's rules comes back with "please repeat", and that costs you another appointment.
If nobody can tell you, ask the person who ordered the tests. If the answer is "it does not matter", ask them to note that on the form, so whoever reads the result knows the conditions.
Find your case
| Your number | What it means | What to ask for | First step |
|---|---|---|---|
| Ferritin and iron | Ferritin, the iron in storage, moves slowly. Iron in the blood runs higher in the morning and jumps after iron tablets, so the same woman gets different numbers on different days. | Do I need to fast? Does the draw have to be in the morning? How long after iron tablets should I wait? | Tell the nurse about any iron tablets, a recent period, blood donation or illness. |
| Thyroid: TSH, free thyroid hormones, antibodies | TSH, the signal your brain sends to the thyroid, shifts through the day. Biotin can distort some thyroid tests. A thyroid tablet taken shortly before the draw can change the free hormone level. | Should I take my thyroid tablet before or after the draw? Does biotin matter for this test? | Decide nothing yourself. Say what you take and when, then follow the prescriber or the lab. |
| Oestrogen, FSH, progesterone | These swing through the month. Some are read early in the cycle, progesterone after ovulation, when the egg is released. With irregular periods the calendar stops working. | Which day of my cycle does my clinician want for these? | Write the first day of your last period. If you cannot tell, write that instead of guessing. |
| Cortisol | Cortisol, the stress hormone, is highest in the morning and falls through the day, so the time of the draw is part of the result. | What time must the sample be taken, and does an illness or poor night change that? | Write the exact time of the draw next to the number. |
Tell the nurse what you take, and get it written on the form
The nurse cannot see your kitchen cupboard. She only knows what you say, and the person who later reads your result sees only what is on the form.
Say these out loud and ask for them to be written down:
- Every supplement you take, especially iron and biotin. Biotin is the vitamin in many hair and nail pills, and it can distort some hormone tests.
- A contraceptive or HRT, the hormone replacement some women take around menopause. Both change the hormones being measured.
- Thyroid tablets, and whether you have taken today's dose.
- A recent illness. The body reacts to infection, and some blood markers move with it.
- A recent period or a recent blood donation. Both take iron out of the body.
Never stop, move or skip a prescribed medicine on your own because of a blood test. Say: "I take this, should I do anything differently before the draw?" The prescriber or the lab decides. This article cannot.
The same goes for biotin and iron tablets. Ask the lab how long before the test they want them left out. I have no reliable document that gives a safe window, so any number I wrote would be a guess.
Ask which cycle day to draw on, and what to write if you cannot tell
Oestrogen and progesterone rise and fall through the month. A draw on one day and a draw on another can give different numbers, and both can be correct for the day they were taken.
Some tests are usually asked for early in the cycle. Progesterone is usually asked for after ovulation, because that is when it rises. There is no single correct day for every hormone. The rule depends on the test, the lab and whether your periods are regular. Ask: "Which day of my cycle do you want this drawn on?" and write the answer down.
If your periods are irregular, or you are in your forties and the gaps keep changing, counting from the last period stops telling you where you are. Write the date your last period began, and "cycle length varies" next to it. Do not pick a day to look tidy. A guess on the form looks like a fact to whoever reads it. Irregular cycles in midlife are covered in Can a Blood Test Show Perimenopause? It Depends on Your Age.
If you are on a pill or HRT, say so. The hormones in them add to your own, and the cycle logic above may not apply.
Iron tablets, illness, a period or a donation can change your iron result
Iron tablets push the number up for a while and make a low result look fine. Ferritin is steadier than iron in the blood, but a recent course of tablets can still flatter either one. Ask the lab how long to wait, and tell the nurse when you last took them.
Inflammation raises ferritin. Ferritin is a stored protein that the body also releases when it fights infection or injury, so after a recent illness a low store can read as a comfortable one.
A recent period or a blood donation lowers the stores. The number is true, but it describes a dip you may recover from, not where you usually are. Write both events on the form.
Iron in the blood runs higher in the morning, so an afternoon draw and a morning draw are not directly comparable. That is why the time of the draw goes next to the number.
Knowing the context makes a repeat test less likely, but does not rule one out. For which tests to ask for in the first place, see Blood Tests to Ask For When You Are Tired All the Time.
Write five things next to every number
Take a pen to the results and add this under every line:
- the date
- the time of the draw
- the day of your cycle, or "cannot tell"
- what you took that week, including supplements and the contraceptive or HRT
- the lab's own range, printed beside your number
The range matters because each lab draws its own line, built from the people it has tested. Why that line moves is in Your Blood Test Range Is Built From Other People's Results.
This page makes the next appointment fair. A number with its context can be compared with the next one. A bare number cannot, and a repeat drawn under different conditions tells you nothing about change. Bring the page to the appointment; How to Explain Your Symptoms in a Short Doctor Appointment covers what to say when time is short.
If it does not go to plan
- Nobody at the lab or surgery can tell you the preparation rulesAsk the person who ordered the tests. If they say it does not matter, ask them to note that on the form so the reader knows the conditions.
- Your periods are irregular and you cannot name your cycle dayWrite the date your last period began and "cycle length varies" next to it. Do not guess a day; a guess on the form reads as a fact.
Honestly: what is settled and what is not
I have no reliable document that gives a safe window for leaving out biotin or iron tablets before a test, so I give no number. Ask your lab how long they want. Fasting and timing rules also differ between labs, and I cannot know yours.
What to do with this
- Ask the lab which tests need fasting or a set time of day, and write the answer down
- Tell the nurse about iron, biotin, other supplements, the pill or HRT, thyroid tablets, recent illness, a recent period or donation
- Ask the prescriber or lab before changing anything you take; never decide alone
- Write the date, time of draw, cycle day or "cannot tell", and the lab's own range next to the number
- Ask which cycle day your clinician wants for oestrogen and progesterone
Questions people also ask
Do I need to fast for an iron or hormone blood test?
It depends on the test and on the lab. Some labs ask for fasting for iron in the blood, others do not for any of your tests. Phone the lab and ask which of your tests need it. Their instruction beats any general rule.
Can I drink water before the test?
Many labs allow plain water, but some give other instructions for particular tests. Ask in the same call where you ask about fasting, and follow what the lab tells you.
Can I have the blood test during my period?
Sometimes, and for some hormones the early part of the cycle is exactly when the clinician wants the draw. For others it is the wrong time. Ask which day your clinician wants, and tell the nurse a period has just started or just ended.
Does biotin really affect blood tests?
It can distort some hormone tests, thyroid among them. How long before the test it should be left out is not something I can state. Tell the nurse you take it and ask the lab how long they want; do not stop anything else on your own.
This article is information, not medical advice, diagnosis or treatment. Reference ranges, doses and protocols differ between people and between laboratories. Talk to a qualified clinician before changing anything about your care.