How to Prep for Your Labs

Everyone tells you to fast. Here is the rest, in a few minutes.

A hair supplement can make your thyroid panel look like you have hyperthyroidism.

Not because anything is wrong with your thyroid. Because most hair, skin and nail products contain high-dose biotin, and a large share of laboratory tests are built on chemistry that biotin interferes with directly. The FDA has issued safety communications about it twice. ¹

It is a small detail, but one worth knowing before your blood draw, not after an abnormal result sends you down the wrong path.

Preparing for labs does not need to be complicated. Most of it comes down to what you do in the 48 to 72 hours before your appointment.

72 hrs

to clear high-dose biotin [1]

48 hrs

no alcohol, no hard training

8-12 hrs

fasting, water encouraged

• Fast 8 to 12 hours. Water only. Keep drinking it, right up to the draw. Dehydration concentrates your blood and pushes several markers higher than they really are.
• Skip alcohol for 48 hours. It moves liver enzymes, triglycerides, and glucose.
• Go easy on training for a day or two, and skip exercise entirely the morning of. Hard sessions elevate muscle and kidney markers for days.
• Eat a normal dinner the night before. Nothing unusually heavy, nothing high in sugar, and go light on red meat. All three carry into the next morning.
• Sleep normally. If you fix only one thing, fix this. A week of short sleep lowered testosterone 10 to 15% in healthy young men. ² One rough night moves cortisol and glucose.
• Book a morning slot and arrive unhurried. Several hormones peak early, and rushing is its own variable.

Biotin: stop 72 hours before

Hair, skin and nail supplements and many B-complex products contain 5,000 to 10,000 micrograms. That is enough to push thyroid results, hormone results, and vitamin D falsely high or falsely low. ¹ A standard multivitamin contains far less and is not the issue. If you cannot stop it, say so, and the lab can often use a method that is not affected.

Creatine: pause for a day or two

Creatine does not harm your kidneys. It does make them look worse on paper. Your body converts some of it into creatinine, which is the number used to estimate kidney filtration, so supplementing raises that number without changing anything about how your kidneys work. ³ Pausing briefly avoids a result that looks alarming and is not.

Iron: skip the morning dose

If iron studies are on your panel, skip the tablet for 24 hours. A dose that morning raises serum iron for hours. Keep taking it otherwise, and note that ferritin is unaffected.

The basics above cover everyone. A few panels have one or two extras worth knowing about.

Still having periods

Where you are in your cycle changes hormone results more than almost anything else on this page, and the best day depends on the question.

Progesterone and whether you are ovulating read best in the mid-luteal phase, roughly days 19 to 21, or about a week before your next period is due. FSH, LH and baseline estradiol are usually drawn around day 3. AMH can be done any day.

If your panel covers several of these, there is no single perfect day. Tell your provider roughly where you are in your cycle when you book, and if your cycles have become unpredictable, say that too. It changes how the numbers should be read.

On testosterone or hormone therapy

Injected testosterone peaks roughly 24 to 48 hours after a dose and bottoms out just before the next one. Drawing near the peak versus at trough can produce two results that point toward opposite dose adjustments, so the convention is to measure at trough: the morning of your next injection, before you inject. ⁵ For daily gels and creams, the same logic applies.

Either way, write down the date and time of your last dose and bring it. A hormone number without that detail is hard to interpret.

Inflammatory markers respond to things that have nothing to do with your long-term health. Push the draw if you have been ill in the past couple of weeks, had a vaccination in the last few days, are dealing with a dental infection or a significant injury, or did something unusually hard that your body is still recovering from.

Water is not cheating. Hydrate right up to the draw.

It is the one thing people skip that makes several markers read wrong.

One result is a snapshot. The useful thing is the trend across several, and a trend is only readable if the conditions matched each time.

A panel drawn fasted at 8am on a rest day and one drawn at 2pm after a workout are not really comparable. Any difference might be your biology or might be the circumstances, and nobody will be able to tell which.

So pick a setup and repeat it. Same time of morning, same fasting window, same couple of days beforehand. It costs nothing and it turns a pile of numbers into an actual picture of what is changing.

Preparation matters most on a baseline panel, because it becomes the thing every future result gets measured against. A first draw taken after a hard week, on four hours of sleep, with a hair supplement still in your system is a baseline you will be comparing against for years.

ANUWELL panels start at $79 for 69 biomarkers and run through to 176, all reviewed by a licensed provider who gives you the read and the next step rather than a PDF. Two days of attention is a reasonable price for numbers you can trust.