Weight loss and hair loss: 7 causes and what your blood shows

Weight loss and hair loss often go together, and the pattern is strikingly predictable: hair only reacts 3 to 4 months after the change. So the hair in your shower drain today belongs to the month you started, not to this week.

That delay makes it hard to pin down the cause yourself.

We think this is one of the nastiest side effects of rapid weight loss, precisely because it is visible and because most information about it comes from hair clinics and wig retailers. Below are seven causes, with what blood testing can show for each one.

1. It is usually telogen effluvium

Telogen effluvium is the term for diffuse hair shedding after a physical shock. A large share of the follicles switch to the resting phase at once and then let go. The literature describes shedding from 3 to 4 months after the trigger, which includes undernutrition, surgery and substantial weight loss.

The hair sheds diffusely across the scalp, not in round patches.

That distinction is useful. Round bald patches fit a different picture and belong with a GP, not on a list about weight loss.

2. Too little protein

Hair is made almost entirely of keratin, a protein. On a strict diet, protein intake usually drops along with everything else, while the requirement stays the same or rises. The body then gives priority to organs and muscle over hair growth.

The Dutch nutrition authority Voedingscentrum uses roughly 0.8 grams of protein per kilo of body weight per day as a starting point for adults, with more for older people.

For GLP-1 users there is something extra at play: appetite falls so sharply that protein intake can halve almost unnoticed. In the STEP 1 trial, participants lost an average of 14.9 percent of their body weight over 68 weeks, and that does not happen without a steep drop in what comes in.

3. Iron and ferritin

Iron is the nutrient most often linked to hair shedding. Ferritin gives an impression of iron stores, while serum iron itself swings considerably across the day.

Values below 30 micrograms per litre are more often associated with increased shedding in dermatology literature, although there is no hard cut-off everyone agrees on.

Two people can both land on a ferritin of 28 micrograms per litre and still have different things going on. In one, little iron came in. In the other, inflammation is running alongside, and that lifts ferritin artificially.

So in that second person the real store sits lower than 28. A normal ferritin therefore does not fully rule out low stores.

4. Zinc

Zinc plays a role in building hair tissue and in the hair cycle. After bariatric surgery, zinc deficiency is regularly found in people with hair loss. Two things play a part there: less comes in, and less is absorbed.

On a GLP-1 course there is no malabsorption, but intake does fall steeply.

A zinc test shows where you stand. Supplementing without a demonstrated deficiency is a different matter: there are indications that excessive supplementation can cause problems of its own.

5. Vitamin D

Vitamin D receptors sit in the hair follicle and take part in the hair cycle. Low vitamin D is regularly found in people with diffuse hair shedding, although the direction of that association has not been established.

In the Netherlands a low value in winter is ordinary anyway, and it falls further when you go outside less.

6. The thyroid

An underactive thyroid produces dry, brittle hair and diffuse shedding. It is one of the few causes on this list with a clear treatment route, which is exactly why TSH is almost always included when someone sees a GP about hair loss.

Confusingly, thyroid symptoms and diet symptoms look very much alike. We worked that out in our piece on thyroid and weight loss.

7. The pace of the weight loss itself

Even without a demonstrable deficiency, rapid weight loss on its own can be enough. The body reads a large energy deficit as a stress event, and hair growth is one of the first things it sets aside.

This is the cause with no blood value attached to it.

Honestly, that is the core of this piece. A normal result does not mean nothing is going on. It means the cause was probably the pace.

With telogen effluvium, hair usually recovers within months once the trigger is gone.

Which tests belong here

The table sets out the four tests most often requested for hair loss after weight loss.

What you can do with this

If your hair has been shedding clearly more for about three months and you lost a lot of weight in that period, that fits telogen effluvium. Thuisarts sets out when hair loss is a reason to get in touch, for example with bald patches or an inflamed scalp.

If you want to know whether a deficiency sits underneath it, ferritin, zinc, vitamin D and TSH sit together in the extended panel. Preferably test before you start supplementing yourself, because after that the result can no longer be read.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Malkud S. Telogen effluvium: a review. J Clin Diagn Res, 2015. PMID 26500992.
• Guo EL, Katta R. Diet and hair loss: effects of nutrient deficiency and supplement use. Dermatol Pract Concept, 2017. PMID 28243487.
• Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med, 2021. PMID 33567185.
• Voedingscentrum, protein.
• Thuisarts.nl, I have hair loss.