Hair thinning in your 40s: iron, thyroid, hormones, in that order
Hair thinning in your 40s is usually a lab result before it is a hair problem. The hair in the brush is the last thing the body shows you and the first thing you notice. By the time the part looks wider, something has been short for months. I read it in a fixed order. Iron first, thyroid second, hormones third. Most of the time the answer sits in the first 2.
Why hair is the last to know
Hair grows in cycles. Most of your follicles are in a growing phase at any one time and a small share are resting, and the resting ones fall out. Losing 50 to 100 hairs a day is the normal cost of that cycle. What changes in your 40s is how many follicles the body decides to rest at once.
When the body is under strain it moves hair to the back of the queue. Hair is not needed to keep you alive, so a body that is short on iron, short on thyroid hormone, short on protein, or coping with a shock will push a larger share of follicles into the resting phase. Those hairs let go about 2 to 3 months later. That delay is why people cannot connect the shedding to the cause. The surgery, the crash diet, the flu, the new weight loss injection, the pill you stopped, all of it shows up on the pillow a full season later. Dermatologists call this telogen effluvium. I call it the body sending a bill.
The second kind of thinning is slower and has a shape. The part widens, the crown thins, the ponytail gets thinner while the hairline mostly stays. That is pattern hair loss, and it has a hormonal and inherited story. Roughly 4 in 10 women show some of it by age 50 and about half of men do. Both kinds often run at once in a woman in her 40s.
Iron first
Ferritin is your iron store. It is the number that gets called normal at 15, and for hair, 15 is nowhere near enough. Several studies use a ferritin under 30 to 40 as the line for iron deficiency, and there is no agreed number for hair. A 2022 review that pooled 36 studies and about 10,000 women found that women with hair loss had lower ferritin on average, and that close to 6 in 10 of them were under a cutoff of 30 to 40. That is an association, not proof that iron fixes hair. It is still the cheapest place to start and the most common thing I find.
The reason it is so common in your 40s is in Issue 032. Heavy periods drain iron faster than food replaces it. A woman whose cycle got heavier at 44 can lose her stores over 2 years and watch her hair go with them, while every blood count comes back fine, because ferritin falls long before hemoglobin does.
Men read this part too. A man with low ferritin is a different problem. He is not losing blood every month, so a low number in a man needs a doctor to find out where the iron is going. That is a workup, not a supplement.
Do not take iron on a guess. It constipates, and in a small share of people it builds up where it should not. Get the number, act on it with your doctor, and if you take thyroid medication keep iron at least 4 hours away from it.
Thyroid second
A thyroid that runs low sheds hair all over the head. So does a thyroid that runs high. The hair gets dry and the outer third of the eyebrows thins. If cold hands, a slow bowel and a flat mood came along with it, Issue 034 is written for you.
The test most people get is TSH alone. TSH is the right first test, but on its own it does not tell you why it moved. I ask for free T4 and antibodies with it, and I add free T3 for context, knowing it rarely helps diagnose a low thyroid. The autoimmune version runs in families and often starts years before TSH moves out of range. Iron sits here too. The enzyme that makes thyroid hormone needs iron to work, so low ferritin can drag thyroid function down with it, and the 2 problems can show up together.
One thing to know before the draw. Biotin interferes with thyroid blood tests. It is in most hair, skin and nails supplements at high doses, and it can make thyroid results look wrong on common lab machines. The FDA has warned about this since 2017. Stop it for a few days before the test and tell the lab what you take.
Hair that fell out because of the thyroid comes back after treatment, slowly. Count in months, not weeks.
Hormones third
In the years before menopause estrogen falls and swings while the androgens, the testosterone family, hold their ground. Estrogen is thought to keep hair in the growing phase longer. This is when the pattern kind tends to show, and the part starts to widen. Why it happens is not fully settled. Most women with this pattern have normal androgen levels, and whether androgens drive it is still debated. It also runs in families.
If this started in your 20s or 30s with irregular cycles, acne or hair where you do not want it, that is a PCOS pattern and it deserves its own workup. Issue 009 covers it. A woman who stopped the pill and lost hair 3 months later is usually in the shedding group. If her cycles do not settle, it is worth checking for PCOS.
For the pattern kind there are real treatments and they belong with a dermatologist. Topical minoxidil is approved for women and works for a good share of people who use it daily for months. Low dose oral minoxidil and spironolactone are prescribed off label, so that conversation has to be with a doctor who knows the field. Men have finasteride on top of that. None of these are for pregnancy or for anyone trying to conceive. Hormone therapy is the most effective treatment for hot flashes and a conversation for a menopause trained doctor. It is not a hair loss treatment.
Fast weight loss sheds hair too. In the trials of the injectable weight loss drugs, hair loss was reported by about 3 to 6 percent of people against about 1 percent on placebo. It is thought to be the shedding kind, mostly in women, and it tends to settle once the weight stops dropping, as long as the protein and iron are there.
Hair is the last thing the body spends on and the first thing you see.
What I would do first
Get ferritin and a full iron panel before anything else, and ask for the printed numbers. If ferritin is under 40 and you have a cycle, the conversation with your doctor is about why it is low and how to bring it up, and the hair comes along for the ride.
Ask for a full thyroid panel, meaning TSH, free T4, free T3 and TPO antibodies. Stop any biotin for a few days first and say so at the draw.
Start 30 grams of protein at breakfast this week. Hair is protein. A body that is short on it rests follicles first. Issue 020 covers how.
Write down what happened 3 months ago. An illness, a surgery, a diet, a drug change, a month of grief. If the shedding is the delayed kind, that is the cause, the hair will come back, and the job is to stop the next one.
Look at the shape of it in good light. Diffuse and everywhere means shedding. A widening part with a stable hairline means pattern, and that is a dermatologist conversation sooner rather than later, because pattern loss is easier to hold than to regrow.
When to push harder with your doctor
Go back, and be direct, if the hair is coming out in patches, if the scalp burns, itches or scales, if the loss is sudden and heavy, if your eyebrows are thinning, or if the hair loss came with heavy periods, cold hands, a slow bowel or a low mood. New hair on the face, acne or a deeper voice alongside hair loss on the head needs hormone testing now. A man with low ferritin needs to know where the iron is going. If the shedding has gone on for more than 6 months, ask for a dermatology referral by name. I am not a doctor and I do not diagnose. I read the numbers next to your cycle, your food and your life, and if it needs clinical care I say so and I send you.
FAQ
What ferritin level is needed for hair growth?
There is no agreed number. Studies often use 30 to 40 as the line for iron deficiency. A result of 15 can pass as normal on the lab report and still sit well under that line. Treat the cause of a low one with your doctor.
Can perimenopause cause hair thinning?
Yes. As estrogen falls, the part can widen and the crown can thin, more so if it runs in your family. It often arrives with heavy periods, which drain iron, so 2 causes stack. Check iron and thyroid before you call it hormones.
Does thyroid hair loss grow back?
Usually, once the thyroid is treated and stable. It is slow. Expect months before you see it, and keep an eye on ferritin through that time, because low iron can slow thyroid function too.
Should I take biotin for hair loss?
Only if you are short on it, which is rare. High dose biotin interferes with thyroid blood tests and some other lab tests, so it can hide the very problem causing your hair loss. Stop it for a few days before any blood draw and tell the lab.
What tests should I ask for if my hair is thinning?
Ferritin and a full iron panel, a full thyroid panel including free T3, free T4 and thyroid antibodies, vitamin D and B12. If cycles are irregular or there is hair on the face, ask for testosterone and DHEAS. Ask for the printed results, not a summary.
In good health,
Yvette Rose
Sources
- Treister-Goltzman Y, Yarza S, Peleg R. Iron Deficiency and Nonscarring Alopecia in Women: Systematic Review and Meta-Analysis. Skin Appendage Disorders, 2022.
- American Academy of Dermatology. Do you have hair loss or hair shedding?
- DermNet. Female pattern hair loss.
- Novo Nordisk. Wegovy prescribing information, DailyMed.
- Eli Lilly. Zepbound prescribing information.
- AbbVie. Synthroid prescribing information, DailyMed.
This is general wellness education and not medical advice, diagnosis or treatment. Talk to a qualified professional, especially if you are pregnant, nursing, taking medication or managing a condition.