Hashimoto's thyroiditis: the autoimmune thyroid disease
Hashimoto's is the most common cause of an underactive thyroid in women. What your test results mean, how it is treated, and where diet actually fits.
Reviewed by Emma Carter, Senior Health & Nutrition Editor
If a blood test has shown an underactive thyroid, Hashimoto's is the most likely reason behind it. It is an autoimmune condition: the immune system slowly damages the thyroid until the gland can no longer make enough hormone. The damage is quiet and gradual, which is why most women feel tired, cold and slow for months or years before anyone thinks to test.
Understanding the autoimmune part explains most of what follows. It explains why symptoms creep in instead of arriving, why antibodies can be raised long before your hormone levels move, and why the treatment replaces the hormone rather than fixing the immune system.
What Hashimoto's is
Hashimoto's thyroiditis is long-term autoimmune inflammation of the thyroid. Over time it can leave the gland unable to make enough hormone, which produces an underactive thyroid, or hypothyroidism. In countries where people get enough iodine, Hashimoto's is the main cause of an underactive thyroid (Cureus, 2023).
People use the two words as if they mean the same thing. They do not, and the difference changes what your results mean.
| Hashimoto's | Hypothyroidism | |
|---|---|---|
| What it is | The autoimmune cause | The underactive-thyroid result |
| What shows on tests | Raised thyroid antibodies | High TSH, low free T4 |
| Can you have one without the other? | Antibodies can be raised for years before the thyroid slows | Can have other causes, but Hashimoto's is the usual one |
Why it turns up in women, and when
Around 5 to 10 percent of people have Hashimoto's, and adult women are diagnosed four to ten times more often than men. Diagnosis clusters around age 54 (Frontiers in Endocrinology, 2026), which is also when perimenopause is in full swing. The two produce overlapping symptoms, and thyroid disease is regularly written off as "just hormones."
What raises the odds:
- A family history of thyroid or other autoimmune disease
- Being female, with risk rising through midlife
- Already having another autoimmune condition
- Very high iodine intake, including kelp and seaweed supplements (Frontiers in Endocrinology, 2026)
You cannot change most of that list. Its use is practical: if any of it applies and you have symptoms that fit, ask for a thyroid blood test instead of waiting it out.
The symptoms
Because Hashimoto's works by slowing the thyroid, the symptoms are those of an underactive thyroid, and they build over months (NHS):
- Fatigue and weakness that sleep does not fix
- Weight gain and feeling cold when everyone else is comfortable
- Constipation, dry skin and hair thinning
- Low mood, brain fog and poor concentration
- Heavier or irregular periods
Early on, hormone levels can still be normal even with raised antibodies, so symptoms may be mild or come and go. Occasionally the gland releases a burst of stored hormone as it is damaged, causing a brief overactive phase before things settle. Some women notice a painless swelling at the front of the neck.
How it is diagnosed
Diagnosis combines a thyroid function test (TSH and free T4) with a thyroid antibody test, mainly thyroid peroxidase antibodies (TPOAb). Raised antibodies confirm the autoimmune process. An ultrasound is sometimes added to look at the gland or a swelling, but the diagnosis does not depend on it.
TSH is the sensitive part of the panel. It rises before free T4 falls, because the pituitary pushes harder as the thyroid struggles to keep up. That is why a raised TSH with a normal free T4 is a common first result.
What your results usually mean
| TSH | Free T4 | TPO antibodies | What it usually means | What usually happens next |
|---|---|---|---|---|
| Normal | Normal | Raised | Autoimmune process, thyroid still coping | Monitoring, not medication |
| Mildly high, up to 10 | Normal | Raised | Subclinical hypothyroidism from Hashimoto's | Treatment considered if you have symptoms; otherwise repeat testing |
| Above 10 | Normal or low | Raised | Near-overt or overt underactive thyroid | Levothyroxine recommended under about age 65 to 70 |
| High | Low | Raised or not | Overt hypothyroidism | Levothyroxine, then retesting to settle the dose |
Those treatment thresholds come from the European Thyroid Association, which recommends levothyroxine for people under 65 to 70 once TSH passes 10, and symptom-led decisions below that (European Thyroid Journal, 2013).
Age changes the reading. The normal TSH range widens as you get older, and a mildly raised TSH of 4.0 to 7.0 in someone over 80 is treated as normal aging rather than disease, with a watch-and-wait approach preferred to medication (European Thyroid Journal, 2013).
What raised antibodies mean, and what they don't
This is where most of the worry comes from, so it is worth being blunt. Raised TPO antibodies confirm that the autoimmune process is there. They do not tell you that you need medication, and the size of the number does not tell you how bad things are. Plenty of women carry raised antibodies with a thyroid that works normally for the rest of their lives.
What antibodies do add is a sense of direction. Among people with subclinical hypothyroidism, those with positive TPO antibodies progress to a fully underactive thyroid at about 4.3 percent per year, compared with 2.6 percent for those without (European Thyroid Journal, 2013). Over a decade that difference matters, but in any given year most people stay where they are.
So positive antibodies mean keep testing, not start treating. Antibodies themselves are not usually re-checked to follow progress; repeat testing is aimed at TSH.
How it is treated
When Hashimoto's has made the thyroid underactive, treatment is levothyroxine, a synthetic copy of the hormone the gland can no longer produce. Full replacement works out at roughly 1.5 to 1.8 micrograms per kilogram of body weight a day, though doses often start lower and are adjusted upward (Cureus, 2023). Most women take it for life.
What to expect:
- Retesting. TSH is measured four to 12 weeks after starting treatment, then about every six months until it holds steady, then yearly (Cureus, 2023).
- How to take it. On an empty stomach, 30 to 60 minutes before your first meal, before coffee or tea, and before any other medicine (NHS). That includes iron and calcium supplements, which interfere with absorption; our guide to supplement timing covers the spacing.
- Pace of improvement. Blood tests normalize before you feel normal. Energy, mood and concentration usually lag behind the numbers by weeks.
If your TSH is in range and you still feel unwell, say so rather than quietly adjusting the dose yourself. Low iron, low vitamin D, low B12 and perimenopause all produce the same fog and fatigue, and all of them are checkable. Our hypothyroidism guide goes deeper into dosing and monitoring.
There is no treatment that switches off the autoimmune process. The aim is to keep hormone levels normal and symptoms controlled, which is achievable for most women.
Pregnancy and the year after birth
Thyroid needs rise in pregnancy, and this is one of the few situations where acting early genuinely changes outcomes. If you already take levothyroxine, the dose is usually increased by 25 to 30 percent as soon as pregnancy is confirmed, with TSH checked about every four weeks up to around 20 weeks and again at 30 weeks. The target is a TSH below 2.5 (Geburtshilfe und Frauenheilkunde, 2023).
If you are not on medication but carry TPO antibodies, pregnancy changes the threshold for treating. With antibodies present, levothyroxine is recommended above a TSH of 4.0 and may be appropriate between 2.5 and 4.0, because untreated subclinical hypothyroidism is linked to a higher risk of miscarriage and premature birth (Geburtshilfe und Frauenheilkunde, 2023). Thyroid function is checked again after delivery, since requirements shift back.
The practical version: if you have Hashimoto's and are pregnant or trying, tell your doctor now rather than at the first appointment.
Conditions that can travel with it
Autoimmune conditions cluster. Hashimoto's carries a somewhat higher chance of others, including celiac disease, type 1 diabetes, vitiligo, and pernicious anemia, which affects vitamin B12. This is not a reason to go looking for trouble. It is a reason to mention new or unexplained symptoms rather than filing them under "thyroid."
Where nutrition fits
Diet and supplements affect how you feel. They do not replace medication, and nothing you eat reverses the autoimmune process.
- Selenium is the honest case. Adding selenium reduces TPO and thyroglobulin antibody levels (Frontiers in Endocrinology, 2026), but a lower antibody number has not been shown to mean fewer symptoms or less medication. Do not take high-dose selenium; the gap between enough and too much is narrow.
- Avoid iodine loading. Excess iodine raises the risk of autoimmune thyroid disease, and a high-iodine diet is specifically discouraged in Hashimoto's (Frontiers in Endocrinology, 2026). In practice that means skipping kelp, seaweed capsules and "thyroid support" blends, which often contain far more iodine than food does.
- Iron is worth checking. Low iron worsens the fatigue and hair thinning you may already be blaming on your thyroid, so ask about ferritin and correct a confirmed iron deficiency.
- Correct a confirmed low vitamin D, rather than supplementing blind.
- Be wary of cure diets. Restrictive plans sold as reversing Hashimoto's are not backed by strong evidence, and going gluten-free clearly helps only if you also have celiac disease.
When to see a doctor
Ask for a thyroid test if you have several symptoms of an underactive thyroid that have built up over months, particularly with a family history of thyroid or autoimmune disease. A blood test settles the question quickly, and treatment makes a clear difference when it is needed.
If you already know you carry thyroid antibodies with normal hormone levels, keep up the monitoring your doctor suggests. The thyroid can slow over years, and the point of testing is to catch that shift before it costs you another winter of feeling flat.
Frequently asked questions
What is Hashimoto's thyroiditis?
It is an autoimmune condition where the immune system attacks the thyroid, causing chronic inflammation and, over time, an underactive thyroid. It affects roughly 5 to 10 percent of people, and adult women are diagnosed four to ten times more often than men.
How is Hashimoto's diagnosed?
With thyroid function tests (TSH and free T4) plus thyroid antibody tests, mainly thyroid peroxidase antibodies (TPOAb). Raised antibodies point to Hashimoto's. An ultrasound of the gland is sometimes used too, but it is not needed to make the diagnosis.
Do raised thyroid antibodies mean I need treatment?
Not necessarily. Many women have raised TPO antibodies with a thyroid that still works normally. Treatment is decided by your thyroid hormone levels and symptoms, not by the antibody number. Antibodies do predict how likely the thyroid is to slow down: in subclinical hypothyroidism, about 4.3 percent of antibody-positive people per year progress to an underactive thyroid, against 2.6 percent of antibody-negative people. That is a reason to keep testing, not a reason to start medication.
How is Hashimoto's treated?
When it causes an underactive thyroid, treatment is levothyroxine to replace the missing hormone, usually long term with periodic blood tests. Full replacement is around 1.5 to 1.8 micrograms per kilogram of body weight a day, and it is taken on an empty stomach, 30 to 60 minutes before food, coffee or other medicines. There is no treatment that stops the autoimmune process itself.
Does Hashimoto's affect pregnancy?
Yes, and it needs planning. Levothyroxine requirements rise, so the dose is usually increased by 25 to 30 percent as soon as pregnancy is confirmed, aiming for a TSH below 2.5. Untreated subclinical hypothyroidism in pregnancy is linked to a higher risk of miscarriage and premature birth, so tell your doctor early if you are pregnant or trying.
Do diet and supplements help Hashimoto's?
They do not replace thyroid medication. Selenium lowers thyroid antibody levels, but lower antibodies have not been shown to translate into feeling better. Correcting low iron and low vitamin D helps how you feel. Avoid high-dose iodine, including kelp, since excess iodine raises the risk of autoimmune thyroid disease, and only go gluten-free if you have celiac disease.
References
- Epidemiology, types, causes, clinical presentation, diagnosis and treatment of hypothyroidism · Cureus, 2023
- Hashimoto's thyroiditis: from pathogenesis to clinical management · Frontiers in Endocrinology, 2026
- 2013 ETA Guideline: Management of Subclinical Hypothyroidism · European Thyroid Journal, 2013
- Recommendations of the AGG on How to Treat Thyroid Function Disorders in Pregnancy · Geburtshilfe und Frauenheilkunde, 2023
- Underactive thyroid (hypothyroidism) · NHS
- Levothyroxine · NHS