The night before
Safety first. Don’t stop or skip any prescribed medicine on your own to get “better” numbers. Every hold in this guide is a question for whoever orders your labs, not a rule.
The short list. Some of these start a few days early.
- Biotin (in hair, skin and nail supplements): ask about stopping it. For thyroid tests, the American Thyroid Association says at least 2 days.
- Fasting sugar test: nothing but water for at least 8 hours. On diabetes medicine? Ask ahead how to handle your dose, and ask about your other morning pills too.
- Workouts: no unusually hard or new workout in the week before, if you can.
- Pills: write down every pill and supplement you take.
- At the draw: sit quietly, and don’t pump your fist.
The one sentence that does the most. Tell whoever orders your labs about your supplements, any hard workouts and any recent illness.
Sources: American Thyroid Association, Clinical Thyroidology for the Public (2018); ADA Standards of Care 2026; the sections that follow.
Supplements and daily pills
Biotin
Biotin is in a lot of hair, skin and nail supplements. The daily amount for adults is about 30 micrograms. Even 1 dose of 10 milligrams (10,000 micrograms, over 300 times that) has thrown off some thyroid tests taken within 24 hours. In case reports, biotin made results look like an overactive thyroid.
It can also make some heart attack blood tests read falsely low, and the FDA got a report of a death after one did. As of the FDA’s last update, some test makers had fixed this and others had not.
Safety first. Chest pain? Call 911 anyway, and tell them you take biotin.
High-dose biotin can make some vitamin D tests read falsely high too. In a small study of 6 adults, it added about 9 points.
Good to know: the American Thyroid Association says stop biotin at least 2 days before thyroid tests, and longer for very high doses. If biotin was prescribed to you at a high dose, ask before you stop it.
B12
B12 pills or shots can push your blood B12 up, so the number can look normal. Tell whoever orders your labs what B12 you take, and don’t stop it on your own. If your B12 is borderline, ask whether a second B12 check, called MMA, makes sense. MMA can also run high with kidney disease and with age.
Folate
A blood folate test mostly reflects what you ate or took recently. Many labs ask you to fast for a folate test, so follow your lab’s instructions. Don’t stop folic acid on your own, especially if you’re pregnant or trying.
Iron
Iron pills can bump up an iron test. Ask whether to skip that day’s dose. Iron in your blood changes through the day and after meals, so ask whether your lab wants a morning or fasting draw.
Thyroid pill
If you take a thyroid pill, ask whoever orders your labs whether to take that day’s dose after the draw. Many doctors do it this way.
Vitamin D
Many healthy adults don’t need routine vitamin D tests. If you’re tested, know that vitamin D builds up in your blood over weeks. After a dose change, it takes months to settle, so ask whether a recheck makes sense, and if so, when.
Sources: NIH Office of Dietary Supplements fact sheets (Biotin, Vitamin B12, Folate); FDA, biotin interference with lab tests and troponin tests (2022); American Thyroid Association (2018); Li et al., JAMA 2017; NICE NG239 (2024); Mayo Clinic Laboratories, folate test; Australian Prescriber, iron studies; Nguyen et al., Clin Biochem 2017; Ain et al., Thyroid 1993; Ramasamy, Clin Biochem Rev 2020; Demay et al., Endocrine Society guideline, JCEM 2024.
Fasting, water and coffee
Most cholesterol tests don’t need fasting anymore. Some tests, like a fasting sugar, still do.
A fasting sugar test means nothing but water for at least 8 hours. If you take medicine for diabetes, ask how to handle your dose that morning.
If you’re fasting, have only plain water. It’s fine, and it makes the draw easier. Hold off on these until after the draw:
- coffee
- juice
- gum
- smoking
- exercise
Good to know: if your triglycerides, a type of blood fat, come back at 400 or higher on a non-fasting test, the usual next step is a repeat test after fasting, to get a clearer LDL number. A high triglyceride still needs follow-up.
Sources: 2026 ACC/AHA dyslipidemia guideline; ADA Standards of Care 2026; MedlinePlus, Fasting for a Blood Test (accessed 2026).
The week before
Hard workouts
- Lifting: in one small study, an hour of lifting raised liver tests in men new to lifting, and they stayed up for at least a week.
- Marathon: after a marathon, creatinine, a kidney blood test, went up about 40%. In runners rechecked a day later, it was back near normal.
Creatine and cooked meat
Creatine, a gym supplement, can nudge up creatinine, so your kidney score can look lower even when your kidneys haven’t changed. These studies were in people with healthy kidneys. Tell whoever orders your labs you take it, and know that a low kidney score still needs follow-up. A different kidney test, cystatin C, isn’t thrown off by creatine; you can ask about it. It has its own things that can throw it off.
A big cooked-meat meal in the hours before can raise creatinine and make your kidney number look worse. UK guidelines advise no meat for 12 hours before this test. A low kidney number still needs follow-up.
Being sick, giving blood, scans
- Being sick: ferritin, a test of your iron stores, goes up when you’re sick, so it can look normal even when iron is low. If you’re sick, ask whether to wait.
- Giving blood: if you gave blood recently, tell whoever orders your labs. In a study of US blood donors, iron and hemoglobin stayed lower for months. Donating is safe and needed.
- Blood loss or low iron: recent blood loss or low iron can push A1c, your 3-month blood sugar average, a bit low or high. If it doesn’t fit your other numbers, ask about another sugar test. A high A1c still needs follow-up.
- CT scan with dye: if you had a CT scan with contrast dye in the last few months, tell whoever orders your thyroid labs. European thyroid guidelines say to factor it in. Don’t skip a scan you need.
Urine tests
- Hard exercise can put protein in your urine for a day or 2. If you trained hard the day before a urine test, ask whether to repeat it.
- Vitamin C can hide sugar or blood on a urine dipstick test. If you take it, mention it before a urine test.
Sources: Pettersson et al., Br J Clin Pharmacol 2008; Leckie et al., J Sci Med Sport 2023; Kabiri Naeini et al., BMC Nephrol 2025; Hajj, Cleve Clin J Med 2025; Preiss et al., Ann Clin Biochem 2007; NICE NG203; BC Guidelines, Iron Deficiency; Kiss et al., JAMA 2015; NGSP, factors that interfere with HbA1c; Bednarczuk et al., Eur Thyroid J 2021; Saeed et al., J Fam Pract 2012; Lee et al., J Clin Lab Anal 2017.
Timing for hormone tests
Testosterone (men)
For men, the Endocrine Society says check testosterone in the morning, fasting, and repeat a low result.
- Time of day: in men in their 30s, testosterone at 4 pm ran about 20 to 25% lower than at 8 am.
- Sugar: in one study of men, a big sugary drink before the test dropped testosterone about 25% for a while.
- On testosterone shots: the Endocrine Society suggests testing midway between shots. Check the timing with whoever prescribes it.
PSA (men)
In one study, PSA, a prostate blood test, went up after ejaculation in most men. The authors suggest waiting 2 days before a PSA test.
Thyroid repeats
In one study, TSH, the main thyroid blood test, was about 30% lower just 2 hours later the same morning. If you repeat a thyroid test, ask to go at the same time of day.
Fertility hormones (women)
For fertility checks, FSH and estradiol, 2 hormones from the brain and ovaries, are usually drawn early in the cycle, around day 3. On birth control or hormone therapy, or without periods, ask what timing applies to you.
Sources: Bhasin et al., Endocrine Society guideline, JCEM 2018; Brambilla et al., JCEM 2009; Caronia et al., Clin Endocrinol 2013; Tchetgen et al., Urology 1996; Mirjanic-Azaric et al., Clin Biochem 2015; ASRM ReproductiveFacts, ovarian reserve.
At the draw
- Keep your hand relaxed. Pumping your fist during the draw can make potassium read falsely high. A truly high potassium is serious, so a high result still needs follow-up.
- Sit quietly for a few minutes first. Even standing versus lying down can shift some results a few percent.
- Same lab. Labs use different methods and normal ranges, so I compare results from the same lab.
Sources: Don et al., NEJM 1990; Lippi et al., Clin Chim Acta 2015; Ceriotti, Clin Biochem Rev 2007.
My prep sheet
Fill this in before your visit and bring it to whoever orders your labs.
My supplements and pills
Include anything you buy without a prescription.
| Name | Dose | Last taken |
|---|---|---|
Questions to ask:
- Do I need to fast for these tests, and for how long?
- Should I stop biotin, or any other supplement, before the draw?
- Should I take my morning pills before or after the draw?
- Does the time of day matter for any of these tests?
- If a result is off, could anything on this list explain it? Which results should we repeat, and which need action right away?
This page is for you and your care team. Please don’t post it or send it to me; I can’t review personal lab results.
My log
| Date | Test | Lab | Fasted? | Notes |
|---|---|---|---|---|