Hair coming out in the shower, on the pillow, in your hands. It is frightening in a way that is hard to explain to anyone who has not had it, and the internet's answer is to sell you a gummy.
The single most useful question is one almost nobody asks first: is your hair shedding, or is it thinning? Shedding is more hair leaving than usual, from all over. Thinning is the same amount of hair getting finer and the parting getting wider. They have different causes, different tests and different outcomes, and telling them apart narrows the field before a single tube of blood is drawn.
The second most useful question is what happened to you about three months ago.
Last updated August 14, 2026
Your iron stores, which fall long before a blood count looks abnormal. One thing to know before you read anything else about this: the claim that ferritin must be above 70 for hair to grow is repeated everywhere, and it rests on weaker evidence than its confidence suggests. A controlled study of 381 women found no more iron deficiency in those with hair loss than in those without [1]. Check it anyway, because low iron is common, causes real problems beyond your hair, and is cheap to correct. The next two tests tell you whether to believe the number.
Ferritin has a second job that gets in the way. As well as storing iron, it rises whenever there is inflammation anywhere in your body, so a ferritin that reads normal can be a low one propped up by something else [2]. hs-CRP measures that inflammation, which is what tells you whether the ferritin number can be trusted.
A second reading on your iron, taken a different way. It gives your transferrin saturation, which is how much of your iron-carrying capacity is in use. The useful part is that inflammation does not push this number up the way it pushes ferritin up, so it cannot hide a shortage the same way: a saturation below 20% says too little iron is reaching the bone marrow where red blood cells are made, even when ferritin has not dropped far [3]. Read the two together and you get an answer rather than a maybe.
Both an underactive and an overactive thyroid cause hair loss, and the shedding is usually diffuse rather than patchy [4]. It is one cheap test that covers both directions.
Read with TSH to tell an underactive thyroid from a borderline one, and to catch the opposite problem.
Low vitamin D is associated with hair loss across every common type of it, pooled over 23 studies [5]. The evidence is moderate rather than strong, because those studies show the two travelling together rather than one causing the other. Deficiency is common enough that finding it is useful whatever it is doing to your hair. If yours comes back low, 2,000 IU of D3 is the sensible daily dose.
Catches outright anemia, which ferritin alone will not show you, and gives the wider picture if your periods have been heavy.
A complete read of the thyroid, paired with the most common non-thyroid causes of the very same symptoms. When you are tired, foggy, or cold, this panel separates a thyroid problem from being low on iron or B vitamins, which feels identical.
Worth knowing what this covers and what it does not. It answers the thyroid question completely, including the antibodies that say whether a thyroid problem is autoimmune, and it covers iron with both ferritin and transferrin saturation. It does not include vitamin D, a blood count, or hs-CRP. Vitamin D can be added to it; the other two are cheap to order separately using the links on the right.
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Worth settling before anything else, because a lot of people are frightened by an ordinary amount of hair. Up to about 100 hairs a day is normal, and you will see more of them on a wash day because that is when several days' worth comes away at once. Long hair also makes the same number look like considerably more.
There is one check you can do at home. Take about sixty hairs between finger and thumb and pull gently along their length. Two or fewer coming away is normal [6]. Four or more suggests active shedding. Usefully, it does not matter when you last washed or brushed: the study behind those numbers found no difference either way.
In active telogen effluvium people commonly lose several hundred hairs a day, so this is usually not a subtle difference. If you are counting and unsure, that is itself informative.
Almost every article about hair loss treats it as one thing. It is at least two, and the distinction does more diagnostic work than any blood test.
You can have more than one at once, which is part of why this gets confusing. But if you are shedding rather than thinning, the next section is probably the most useful thing on this page.
Telogen effluvium is a mass exit, and what makes it so disorienting is the delay: the shedding starts two to three months after whatever caused it [4], so by the time you notice, the cause has been and gone and you are looking for it in the wrong week.
Work backwards a season. A serious illness or high fever. Surgery. Childbirth. A crash diet or rapid weight loss. Stopping or starting hormonal contraception. A few months of real strain. Any of those can do it, and none of them will be happening when the hair starts falling.
This matters because the outcome is good and most of the advice you will be sold is unnecessary. Acute telogen effluvium resolves on its own in the large majority of cases [4]. Shedding typically runs three to six months, regrowth begins as it slows, and density recovers over the following year. No supplement shortens that.
Two things worth doing anyway: rule out the thyroid and iron, because they cause the same picture, and eat enough. Rapid weight loss and not eating enough protein are among the few nutritional causes with a real mechanism behind them [4].
The most common question asked about hair loss is which vitamin is missing. For most people, the answer is none. The nutritional causes with a real mechanism behind them are iron, vitamin D and not eating enough protein, and two of those are on the test list above because they are worth measuring rather than guessing at. Everything below is what gets sold to fill the gap.
If you take a biotin supplement, stop it a few days before any blood test. Biotin throws thyroid results high or low, so a distorted result does not announce itself as wrong.
Some presentations are not a blood-test question.
One question worth expecting, because people search it in exactly these words: why is my hair falling out when my blood tests are normal? Often because normal is a wide band drawn from the whole population, and low-normal iron is still low for you. Sometimes because the answer was never in your blood, and the cause was a season ago. If you have your numbers, seeing where each one sits against the range the evidence supports is the next thing worth doing.