Hair loss: causes and which tests to ask for

There is a moment when you stop thinking “it is normal” and start counting hairs on the pillow. The first reaction is usually one of two: ask for a blood test of everything, or buy a shampoo. Both skip the step that gives the most information, which is dating the shedding and looking at what came before.

This text explains the types of hair loss worth telling apart, the three tests that actually answer questions, when the dermatologist comes before the lab, and how to log the shedding so the appointment does not end in “let’s see how it evolves”.

First, what kind of hair loss it is

Losing hair is normal: the figure dermatologists usually quote is 50 to 100 hairs a day, because each follicle cycles through growth, rest and shedding. What marks a problem is not only the amount, it is the pattern. The American Academy of Dermatology separates several types with different causes and management,^[1] and for someone starting to look, three groups are enough:

The timeline: why hair falls out two or three months later

The most common diffuse shedding is called telogen effluvium. The mechanism is simple: a strong trigger pushes many follicles at once into the resting phase, and resting hair falls out about two or three months later.^[2] That is why the shedding seems to come out of nowhere: by the time you notice it, the trigger is over and nobody connects the two.

The usual triggers are a high fever or a significant infection (including the respiratory ones that knock you out for a week), surgery, childbirth, a strict diet or rapid weight loss, starting or stopping some medicines, and a period of intense stress. In most cases the episode closes on its own within months, when the follicles start growing again.^[2] What can prolong it is having, in addition, low iron or a slow thyroid: that is why the three tests below are worth it even when the trigger is obvious.

The practical exercise: take the date you started noticing the shedding and look at what happened two, three and four months earlier. If there is a trigger there, you already have the main hypothesis, and the appointment changes from “what is wrong with me?” to “does this fit what I went through in July?”.

The three tests that answer questions

With diffuse shedding and no warning signs, the lab serves three concrete questions. A panel “of everything” does not add answers, it adds flagged values that distract.

Ferritin: iron stores

It is the most frequent question and the one most often left unasked, because a blood count gets ordered and ferritin does not. Haemoglobin is the last thing to drop when iron is missing; the stores empty first, and that is what ferritin measures. The WHO sets the cut-off for depleted stores in healthy adults at 15 µg/L,^[4] and in clinical practice many doctors treat any value below 30 µg/L as suggestive of deficiency.^[5] The link between low iron and hair loss has been described for years, although the evidence is not enough to recommend measuring iron in everyone with hair loss: it is measured when the picture suggests it,^[3] and the picture suggests it with tiredness, breathlessness on effort, brittle nails, heavy periods or a low-iron diet.

TSH: the thyroid

Thin, brittle, shedding hair is one of the signs guidelines associate with a thyroid working slowly, together with tiredness, feeling cold, dry skin, constipation and weight changes.^[6] A single test, TSH, answers the first question; if it is off, your doctor will decide whether to add free T4 and antibodies. Thyroid-related shedding is usually gradual and accompanied: if hair is the only thing that changed, the thyroid is less likely, but the test is cheap and rules it out.

Complete blood count: the overall picture

It confirms or rules out anaemia and gives context to the rest. If there is anaemia along with low ferritin, the iron hypothesis is reinforced; if haemoglobin is fine and ferritin is low, it is still iron, only at an earlier stage.

And vitamins, zinc, biotin

The most cited review on micronutrients and hair loss concludes something unpopular: outside clear deficiencies (iron, in some cases vitamin D or zinc in very restrictive diets), the evidence that a supplement slows shedding in people without a deficiency is weak, and biotin only helps when it is lacking, which is rare.^[7] A practical detail: high-dose biotin can interfere with several lab tests, including TSH. If you take supplements, tell the lab and your doctor, and stop them in the days before if you are told to.

When hormones enter the list

Hormone tests are not ordered “for the hair loss”: they are ordered for what accompanies it. In women, hair thinning at the crown together with acne, facial or body hair and irregular or absent cycles points to androgen excess, and the international guideline on polycystic ovary syndrome includes male-pattern hair loss among the clinical signs to assess.^[8] In that case the doctor decides what to measure and on which cycle day, because several hormones change over the month and a value without a cycle date cannot be read. In men, gradual thinning at the crown and temples with a family history is almost always androgenetic pattern, and the diagnosis is clinical: the dermatologist sees it without blood.

How to log hair loss so the appointment is useful

“I am losing a lot of hair” is hard to assess in a twenty-minute appointment. Four things make it measurable:

1. Start date of the shedding, even approximate, and what happened in the two to four months before: fever, infection, surgery, childbirth, diet, new medicines, stress.
2. Photos of the crown, front hairline and parting, one a month, same light and distance. It is the only way to see whether it improves.
3. How much falls out: a home test dermatologists use is counting the hairs left in your hand after running your fingers through a lock from the root; if several come away each time, the shedding is active.
4. What accompanies it: tiredness, feeling cold, weight changes, cycle, diet, supplements. It is what decides which tests to order.

With this and the three tests, the first appointment usually closes with a hypothesis and a date to review, instead of “let’s see how it goes”.