Hashimoto's disease: symptoms, TPO antibodies and how it differs from hypothyroidism

Hashimoto's disease is an autoimmune condition: the immune system makes antibodies (immune proteins) against the thyroid and slowly damages it, and over years that damage can leave the gland unable to make enough hormone. ¹ It is the most common cause of an underactive thyroid in the US, but having Hashimoto's and having an underactive thyroid are two different things. ² The first is read from a TPO antibody test (thyroid peroxidase antibodies, aimed at an enzyme the thyroid uses to build hormone); the second is read from TSH (thyroid-stimulating hormone, the pituitary's push on the thyroid) and free T4 (the main thyroid hormone in the blood). What to do next: if a report shows positive TPO antibodies, bring it to your clinician together with a TSH and free T4 from the same period, and ask which stage those two numbers put you in.

What happens in Hashimoto's disease

In Hashimoto's disease, white blood cells called lymphocytes move into the thyroid and settle between the small sacs (follicles) where hormone is made. ⁶ The immune system also makes antibodies against two thyroid proteins: thyroid peroxidase (TPO), the enzyme that attaches iodine to build hormone, and thyroglobulin (Tg), the protein the hormone is built on. ³ Over years, the inflammation can scar or shrink the gland. Most forms of Hashimoto's end in an underactive thyroid, although at first people can have normal thyroid levels or even a brief overactive phase. ⁶

Other names for the same condition are Hashimoto's thyroiditis, chronic lymphocytic thyroiditis and autoimmune thyroiditis. ¹ Thyroiditis means thyroid inflammation.

Who gets Hashimoto's and how common it is

Hashimoto's disease is 4 to 10 times more common in women than in men and most often develops between ages 30 and 50, though teenagers and young women can be affected. ¹ Having relatives with Hashimoto's raises the risk. ¹ The US total is not known precisely, partly because so many people never have symptoms, but hypothyroidism, which Hashimoto's causes more often than anything else, affects about 5 in 100 Americans. ¹

The causes are not fully known. Genes play a part, and viruses such as hepatitis C may contribute. ¹ People who already have one autoimmune condition are more likely to develop another: NIDDK names celiac disease, lupus, rheumatoid arthritis, Sjögren's syndrome and type 1 diabetes. ¹ Pernicious anemia, an autoimmune cause of vitamin B12 deficiency, is also more common in people with an underactive thyroid. ¹³

Hashitoxicosis: when Hashimoto's briefly looks overactive

Early on, inflammation can break open thyroid follicles and spill stored hormone into the blood. For a few weeks the picture can look like an overactive thyroid, with a low TSH, a fast heart and shakiness, before the gland settles and later slows down. ⁶ This phase is called hashitoxicosis. It looks like hyperthyroidism on a panel, which is one reason a clinician looks for the cause before treating any low TSH.

How Hashimoto's disease is diagnosed

There is no single test that settles it. The diagnosis rests on a medical history, a neck exam and blood tests for TSH, free T4 and TPO antibodies, and an ultrasound may be used if antibodies are negative but Hashimoto's is still suspected. ¹ Caturegli and colleagues describe the diagnosis as antibodies to thyroid proteins plus a darker (hypoechoic) pattern on ultrasound in someone with fitting clinical features. ⁶

The markers to check, and what each one adds

Hashimoto's touches more than the thyroid panel, because autoimmune conditions cluster and an underactive thyroid can change other results. The table sorts the tests into the ones that answer the thyroid question and the add-ons a clinician may consider when symptoms or history point that way.

Free T4 cutoffs vary by lab and method, so the ranges above are a rough guide only; the thyroid panel guide walks through how TSH, free T4 and free T3 move together, and the autoimmune antibodies guide covers what a positive antibody test does and does not mean across the body.

How the results read together

TPO antibodies over 200, 900 or 1,000: what the number means

People search for these numbers because they look alarming next to a range that tops out at 34. A TPO antibody result of 200, 900, 1,000 or 1,300 IU/mL is positive on every common assay, and all of them answer the same question with the same answer: yes, there is an autoimmune process aimed at the thyroid. ⁷ The size of the number is not a severity grade. It does not tell anyone how tired a person is, how much hormone the gland is making or what dose of medicine is needed.

What higher antibodies do change is the forecast. In the Whickham survey, which followed a random sample of adults in northern England for 20 years, women with positive antibodies alone had 8 times the odds of developing an underactive thyroid, and women with positive antibodies plus a raised TSH had 38 times the odds, compared with women with neither. ⁵ A review in American Family Physician adds that higher TPO levels raise the risk of progressing. ¹¹ That is why the follow-up is a TSH every so often, rather than chasing the antibody number.

Repeating and monitoring your thyroid antibody levels is not needed.American Thyroid Association, Hashimoto's thyroiditis patient information ²

Very low readings sit at the other end. A TPO antibody result of 8 or 10 IU/mL is negative on Labcorp's assay, though a lab with a lower cutoff may flag it. ⁷ Labcorp also notes that a small share of healthy people, about 3%, carry the antibody, and that people with lupus, rheumatoid arthritis, Sjögren's syndrome or pernicious anemia can test positive too. ⁷

Look-alikes and the test that tells them apart

Managing Hashimoto's: what the evidence shows

Care for Hashimoto's depends on whether the thyroid is underactive. Without hypothyroidism, clinicians usually monitor symptoms and hormone levels and do not start medicine; the American Thyroid Association says people with high antibodies and normal hormone levels do not require thyroid hormone. ¹² Once hypothyroidism develops, the standard treatment is levothyroxine, a synthetic copy of the T4 the gland makes, and an American Thyroid Association task force concluded that levothyroxine should remain the standard of care. ¹⁴ Most people with hypothyroidism from Hashimoto's need it for life. ² The dose is set and adjusted by a clinician from repeat TSH results; the levothyroxine guide explains how TSH is read on treatment.

Selenium deserves a closer look because it is the supplement asked about most. The thyroid needs selenium to make and recycle its hormone, and a 2024 meta-analysis in the journal Thyroid found selenium lowered TPO antibodies and modestly lowered TSH in people not on thyroid hormone. ⁸ But in the six trial groups that measured well-being, two found an improvement and four found no difference, and the authors wrote that the clinical relevance of lowering TPO antibodies warrants further investigation. ⁸ The selenium guide covers the ceiling where selenium turns harmful and why more is not better.

Vitamin D comes up next. Low vitamin D is common, and NIDDK says researchers are studying vitamin D in Hashimoto's but there is no specific guidance yet. ¹ A measured low level is its own question for a clinician, covered in the vitamin D guide and on the vitamin D deficiency page, rather than a Hashimoto's treatment.

Diet and Hashimoto's

Diet searches are among the most common for Hashimoto's, and the honest answer is short. Two diet points have evidence behind them. First, iodine: the thyroid needs it, but people with Hashimoto's can be sensitive to large amounts, and NIDDK warns that iodine-rich foods and supplements such as kelp and other seaweed can cause or worsen hypothyroidism. ¹ Some cough syrups also contain iodine. ¹ Second, gluten: a gluten-free diet treats celiac disease, and celiac disease is found in about 1.6% of people with autoimmune thyroid disease, about 1 in 62. ⁹ Outside those two, no specific Hashimoto's diet has been shown to slow the disease. ¹¹⁰

Hashimoto's, pregnancy and planning a pregnancy

Pregnancy changes thyroid needs. Thyroid hormone requirements can rise as early as the fourth week of pregnancy, and in people already on levothyroxine, clinicians usually check TSH at the first prenatal visit and then about every four weeks. ¹¹ The American Thyroid Association updated its pregnancy guideline in 2026, covering thyroid autoimmunity, hypothyroidism and the postpartum period. ¹² Anyone with positive TPO antibodies who is pregnant or planning a pregnancy should tell the clinician early, because thyroid testing in pregnancy follows its own rules and its own timetable.

Monitoring and retesting

One reading is a snapshot. The trends guide explains why two or three TSH results over time say more than any single number.

When to see a doctor

Book a routine visit for new tiredness, weight change or period changes with positive antibodies, for any pregnancy or plan to conceive, and before starting any thyroid, iodine or selenium product. The thyroid panel guide lists the patterns worth raising.

Questions to ask your clinician

• Do my TSH and free T4 show that my thyroid is working normally, or is it underactive (subclinical or overt)?
• Given my antibody result, how often should my TSH be checked?
• Would a celiac blood test, B12 or ferritin make sense for me?
• Is a thyroid ultrasound useful for me, for example if I have a goiter or a lump?
• Are any of my supplements, such as biotin, kelp or selenium, likely to affect my results or my thyroid?
• If I am pregnant or planning a pregnancy, how does that change my testing?

For the neighbouring pages: the hypothyroidism guide owns the TSH numbers (what a TSH of 5, 7 or 10 means) and treatment decisions, and the hyperthyroidism guide covers a low TSH and Graves' disease. The TPO antibody page goes deeper on the test itself.

This guide is general education, not medical advice or a diagnosis. The clinician who knows your history reads your results and makes decisions with you.

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