For about three months now the shower drain has been asking for attention halfway through the wash. The ponytail takes two extra turns of the elastic. Then someone said the line: “that’s your thyroid, go get tested.” And you came here to find out whether that is a hunch or whether it holds up.
It holds up. But the part of this story that matters most is almost never the part you were told.
The follicle reads thyroid hormone
The hair follicle is one of the fastest-dividing structures in the human body. To keep that pace it depends on basal metabolism, and what calibrates basal metabolism in practically every tissue is thyroid hormone.
This is not a vague influence. The human follicle has receptors for T3, and studies on cultured follicles show that T3 and T4 change the length of the growth phase and the rate at which matrix cells enter programmed cell death.
Under normal conditions, somewhere between 10% and 15% of your hairs are in the resting phase at any given moment, each on its own schedule, and you lose 50 to 100 a day without noticing. When a systemic signal disrupts the cycle, a large block of follicles goes into resting phase at the same time. Two to three months later, that block lets go together.
That is why thyroid-related loss is diffuse: it thins the whole head, it shows up in the width of your parting and the volume of your ponytail, and it does not create patches with a defined edge. A round patch with a sharp border is a different conversation, and that conversation belongs to a dermatologist.
Too little hormone and too much do different things
Both ends cause shedding, but the picture around it is not the same. It is that surrounding picture that points to the right test.
| Hypothyroidism (too little hormone) | Hyperthyroidism (too much hormone) | |
|---|---|---|
| In the hair | Diffuse shedding, drier, coarser hair with no shine and no body | Diffuse shedding, hair too fine and too soft to hold a style |
| In skin and nails | Dry skin, brittle and ridged nails | Warm, moist skin, nails lifting off the bed |
| In the body | Fatigue, weight gain with no change in diet, cold intolerance, constipation, slow recall | Weight loss eating the same or more, feeling hot, racing heart, fine hand tremor, insomnia, irritability |
| Body hair | Thinning of the outer third of the eyebrow, near the temple | Less characteristic |
| Pace | Settles in slowly, over months, and gets filed under “it’s my age” or “it’s stress” | Usually gets attention faster, because of the cardiac symptoms |
None of these signs belongs to the thyroid alone. Thinning at the outer tip of the eyebrow, for example, is classic, but it does not happen to everyone and it also shows up for other reasons, including years of repeated plucking or waxing.
Shedding on its own is rarely the thyroid
Here is the first belief worth taking apart: that hair falling out is reason enough, by itself, to suspect the thyroid.
What raises the suspicion is the combination. Shedding with fatigue, with weight change you cannot explain, with fragile nails, with feeling cold or hot in a way that is not normal for you. Shedding on its own, with no other symptom, in someone with no family history, has other more likely explanations: low iron, a stressful event dated two to three months earlier, the postpartum period, a new medication, or an androgenetic pattern that had been advancing slowly all along.
That does not mean skipping the test. TSH (thyroid-stimulating hormone) is a cheap test, available both through SUS, Brazil’s public health system, and through private insurance, and hypothyroidism is considerably more common in women, with a frequency that rises with age. It only means not parking there. Investigating one cause at a time is the most efficient way to lose a year.
What to ask for, and why not TSH alone
The basic pair is TSH and free T4. TSH is the hormone the pituitary sends out to demand production from the thyroid, so it rises when the gland is delivering too little and falls when it is delivering too much. It is sensitive, but on its own it does not tell the whole story.
When TSH comes back abnormal, or when there is thyroid disease in the family, the doctor will often add anti-TPO (antithyroid peroxidase antibody), which indicates an autoimmune process, the most common cause of hypothyroidism in Brazil, where salt is iodised.
And it is worth asking, in the same request, for the tests that cover what produces exactly the same shedding picture: complete blood count, ferritin and vitamin D. More than one cause coexisting is the rule, not the exception. Correcting the thyroid and leaving ferritin at 10 gets you half a solved problem.
One practical detail almost nobody mentions: high-dose biotin supplements interfere with thyroid tests. Many laboratory assays use biotin in the chemistry of the measurement itself, and an excess in the blood distorts the result, upwards or downwards depending on the method. If you take a hair and nail capsule, read the label, tell the lab and your doctor, and ask how long to stop before the blood draw. The usual advice is at least two to three days. A falsely abnormal TSH buys you an investigation and an anxiety that never needed to exist.
The blood test normalises long before the mirror does
This is the point of the whole article, and it is where most people give up.
You start treatment, repeat the test in six or eight weeks, the number comes back into range, the fatigue lifts, and the hair keeps falling. The hasty conclusion is that it was not the thyroid after all, or that the medication is useless, or that the doctor got it wrong.
Do the follicle arithmetic. The hairs that have already entered resting phase will let go regardless, because that decision was made two to three months ago. After that, the follicle has to restart the growth phase. And hair grows at roughly 1 cm per month. Three months of growth is 3 cm of new hair: short, fine and standing up along the parting. That is a sign, but it is not density yet.
So what you can reasonably expect is that the first real sign shows up as short new hairs coming through, and that a noticeable change in volume takes months, counted not from the day you started the medication but from the day your blood test stabilised. Transient shedding, in fact, is described at the start of levothyroxine treatment. It is not a reason to stop anything on your own.
It is in that gap of months between the corrected blood test and the mirror catching up that the shelf of anti-shedding ampoules (the single-dose vials sold in every Brazilian pharmacy) sells best. The product did nothing, and it takes the credit for what the cycle would have done on its own.
The reverse is worth saying too: levothyroxine is not a hair supplement. The decision to treat mild hypothyroidism, with borderline numbers, belongs to the doctor, weighing symptoms, antibodies and a repeat test, because excess hormone has a cost for bone and heart. Hair is not a sufficient argument for medicating.
Why no cosmetic corrects this
Shampoo, tonic and ampoule act on two things: the shaft, which is keratin already formed and no longer alive, and the surface of the scalp. The order to enter resting phase does not come from either of those places. It comes from the blood.
There is no topical active that replaces a hormone. Anyone promising that is selling you back the time of your own cycle.
What hair therapy work does, being honest about the size of its role, is something else: keep the scalp in a condition to receive the regrowth, because inflamed, flaking skin, or skin loaded with sebum and residue, gets in the way of new hair; remove the aggravating factors that stack on top, such as tension from tightly pulled styles, chemical services on already fragile hair, and aggressive brushing over the short new hairs coming in; look after the quality of the shaft, which in hypothyroidism turns drier and duller and does not improve just because the blood test improved; and measure, so you know whether you are regrowing or not.
The order matters: doctor first, hair therapy alongside and afterwards.
When to stop reading and see a doctor
Seek medical care without going through any salon if there is a lump or swelling in the neck, difficulty swallowing, new and persistent hoarseness, a racing heart at rest, or weight loss you cannot explain.
And see a dermatologist if the shedding comes with a sharply bordered patch, redness, flaking with strong itching, a sore, crusting or pain on the scalp. The thyroid does not make a round hole.
What to do with what you have just read
Three things, today. Write down the date you noticed the shedding and what happened two to three months before it. Make an honest list of your other symptoms, including the ones you had been putting down to your routine, the heat or your age. And book a general practitioner or an endocrinologist, asking for TSH, free T4, complete blood count, ferritin and vitamin D, with that list in your hand.
If you are already in treatment, note the date your test came into range. That is the date the hair clock starts from, not the first capsule.
If you would like to talk about your case
The trichoscope hair assessment shows the scalp magnified, with the screen turned towards you, and you follow the trichoscopy together with the professional. It records the density and the calibre of your hair as it is today, which is the baseline you will want to have when you need to know whether anything has regrown; it tells diffuse shedding apart from miniaturisation, two different prognoses the mirror does not separate; and it flags when the case belongs to a doctor, which in this particular story is the most likely outcome, and is an expected result of the assessment rather than a failure of it.
R$200 (Brazilian reais), credited in full towards the protocol. Bring your blood tests, if you already have them. We measure again during or at the last session and compare with the image from the first day. At no extra cost. We are in Tatuapé, in the East Zone of São Paulo, close to the Carrão and Tatuapé metro stations, open Tuesday to Saturday from 9am to 6pm, by appointment. Tuesday is the day reserved for scalp therapy. We also offer service in English.