Hypothyroidism itself is not classified as an autoimmune disease, but in about 90% of adults in iodine-sufficient countries, it is caused by one. The underlying condition is called Hashimoto’s thyroiditis, an autoimmune disorder in which the immune system attacks and gradually destroys the thyroid gland. So while hypothyroidism is the result (an underactive thyroid), the autoimmune process is what drives most cases.
Why the Distinction Matters
Hypothyroidism simply means your thyroid gland doesn’t produce enough hormones to meet your body’s needs. It’s a description of what’s happening, not why. The “why” can be autoimmune, surgical, medication-related, or something else entirely. When your doctor says you have hypothyroidism, that diagnosis alone doesn’t tell you whether your immune system is involved.
That said, the odds strongly favor an autoimmune cause. Globally, Hashimoto’s thyroiditis affects an estimated 7.5% of the population, and in countries where iodine deficiency has been addressed through salt iodization programs, autoimmune thyroiditis has become the dominant cause of hypothyroidism. Even in countries like India, where iodine deficiency was historically the leading cause, autoimmune thyroiditis has overtaken it.
How Hashimoto’s Destroys the Thyroid
In Hashimoto’s thyroiditis, the immune system misidentifies thyroid tissue as a threat and mounts a sustained attack using two coordinated systems. One involves immune cells that directly infiltrate the thyroid gland. Cytotoxic T cells burrow into thyroid tissue, causing inflammation and destroying the cells that produce thyroid hormones. Helper T cells amplify the assault by activating additional immune players. Regulatory T cells, which normally keep the immune system in check and prevent it from attacking the body’s own tissues, are suppressed in Hashimoto’s. This removes a critical safety brake.
The other arm of the attack is antibody-driven. The immune system produces antibodies that target specific proteins in the thyroid. The most common is an antibody against thyroid peroxidase, an enzyme the thyroid needs to make its hormones. Over 90% of people with Hashimoto’s carry this antibody. Another antibody targets thyroglobulin, a protein the thyroid uses as a building block, and is found in 50% to 80% of cases. A third type of antibody blocks the receptor that normally receives signals from the brain telling the thyroid to produce hormones.
Together, these immune mechanisms cause progressive scarring (fibrosis) of the thyroid gland. Over months or years, enough tissue is destroyed that the gland can no longer keep up with the body’s demand for thyroid hormones, and hypothyroidism develops.
Non-Autoimmune Causes of Hypothyroidism
The remaining roughly 10% of hypothyroidism cases have nothing to do with the immune system. These include:
- Thyroid surgery: Partial or complete removal of the thyroid, often for cancer or large nodules, reduces or eliminates hormone production.
- Radiation therapy: Radiation to the head or neck for cancer treatment can damage the thyroid.
- Medications: Certain drugs, including lithium (used for bipolar disorder), can suppress thyroid function.
- Iodine deficiency: The thyroid needs iodine to build its hormones. In regions without iodized salt, this remains a significant cause.
- Congenital conditions: Some babies are born with a thyroid that doesn’t work properly or is missing entirely.
- Pituitary problems: Rarely, the pituitary gland fails to send the hormonal signal that tells the thyroid to produce hormones.
- Pregnancy: Some people develop hypothyroidism during or after pregnancy, which can be temporary or permanent.
How to Know If Your Hypothyroidism Is Autoimmune
The symptoms of hypothyroidism are the same regardless of the cause: fatigue, weight gain, cold intolerance, constipation, dry skin, thinning hair, heavy or irregular periods, joint and muscle pain, and a slowed heart rate. These symptoms develop gradually, and many people with Hashimoto’s have no symptoms at all in the early stages.
One physical sign that points toward an autoimmune cause is a goiter, an enlarged thyroid that makes the front of the neck look swollen. It may create a feeling of fullness in the throat, though it’s typically painless. Not everyone with Hashimoto’s develops a goiter, but when one appears alongside hypothyroidism, it raises suspicion.
The definitive way to confirm an autoimmune cause is a blood test for thyroid peroxidase antibodies (TPO antibodies). This test has a specificity of about 97%, meaning a positive result very reliably points to autoimmune thyroid disease. Your doctor may also check for thyroglobulin antibodies. If these antibodies are present alongside an elevated TSH level, the diagnosis of Hashimoto’s thyroiditis is straightforward.
Does the Cause Change Treatment?
For the hypothyroidism itself, not really. Whether your thyroid was destroyed by your immune system or removed by a surgeon, the treatment is the same: daily thyroid hormone replacement. The dose is calibrated to your body weight and how much thyroid function you still have, then adjusted over time based on blood work. Once your levels stabilize, you’ll continue the medication long-term, with periodic checks to make sure the dose stays right.
Where the cause does matter is in what you should watch for next. Hashimoto’s sometimes goes through an early phase where the damaged thyroid releases a burst of stored hormones into the bloodstream, temporarily causing symptoms of an overactive thyroid (rapid heartbeat, anxiety, tremor) before settling into permanent underactivity.
Linked Autoimmune Conditions
Knowing your hypothyroidism is autoimmune also matters because autoimmune diseases tend to cluster. About 14% of people with autoimmune thyroid disease have at least one additional autoimmune condition. The most common companions are Sjögren’s syndrome (which causes dry eyes and dry mouth) and lupus. Celiac disease, rheumatoid arthritis, type 1 diabetes, and a form of vitamin B12 deficiency also appear more frequently in people with Hashimoto’s than in the general population.
People with rheumatoid arthritis, for example, are two to three times more likely to develop autoimmune thyroid disease. These overlapping conditions are sometimes grouped under the term autoimmune polyendocrine syndromes. If you’ve been diagnosed with Hashimoto’s and you develop new, unexplained symptoms like persistent joint pain, dry eyes, or digestive problems with certain grains, it’s worth exploring whether another autoimmune condition is involved.