Hashimoto’s disease develops when your immune system mistakenly attacks your thyroid gland, gradually destroying the tissue that produces thyroid hormones. There isn’t a single cause. Instead, it results from a combination of genetic vulnerability and environmental triggers that, together, push the immune system into attacking the thyroid. It affects roughly 7.5% of adults worldwide, and women are about four times more likely to develop it than men.
Genetics Set the Stage
Your genes are the single largest factor determining whether you’ll develop Hashimoto’s. Statistical modeling suggests that around 79% of the predisposition to autoimmune thyroid disease comes from genetic factors. That doesn’t mean genes alone cause the disease, but they create the conditions under which triggers can tip the immune system into action.
Two gene regions account for the bulk of this inherited risk. The HLA region, part of the immune system’s toolkit for distinguishing self from non-self, has been linked to autoimmune thyroid disease for over two decades. The second major player is the CTLA-4 gene, which helps regulate how aggressively immune cells respond. Variants in CTLA-4 increase the relative risk of developing Hashimoto’s by about 1.45 times. A third gene of interest codes for thyroglobulin, a protein the thyroid uses to manufacture hormones. Specific variations in this gene on chromosome 8 have also been linked to increased susceptibility.
If you have a parent or sibling with Hashimoto’s or another autoimmune thyroid condition, your risk is substantially higher than the general population. But genetics alone don’t seal your fate. Something in your environment or biology still has to pull the trigger.
How the Immune System Destroys the Thyroid
In Hashimoto’s, thyroid cells are gradually replaced by immune cells that infiltrate the gland. The destruction happens through a specific process: immune signaling molecules produced during inflammation cause thyroid cells to display a “self-destruct” receptor on their surface. When that receptor is activated, the thyroid cells die off in an orderly process called apoptosis. The inflammatory molecule interleukin-1β, released during thyroid inflammation, is one of the key signals that ramps up this self-destruct pathway.
T cells (the immune system’s targeted attackers) initiate and amplify the autoimmune response, while B cells produce antibodies against two thyroid proteins: thyroid peroxidase (TPO) and thyroglobulin. These antibodies are measurable in blood tests and serve as the primary way Hashimoto’s is diagnosed. Normal TPO antibody levels are below 5.6 IU/mL, and thyroglobulin antibodies should be below 4 IU/mL. Levels above these thresholds, combined with symptoms and thyroid function tests, confirm the diagnosis.
Iodine: Too Much Can Be a Problem
Iodine is essential for making thyroid hormones, and the WHO recommends a daily intake of about 150 to 300 micrograms. Most healthy people can tolerate up to 2,000 micrograms per day without trouble, as long as they don’t have underlying thyroid inflammation. But in people who are genetically susceptible or already have mild, undetected thyroiditis, excessive iodine can provoke or worsen the autoimmune response.
Seaweed is one of the most concentrated dietary sources of iodine and can push intake well beyond safe levels, especially for pregnant women and people with existing thyroid autoimmunity. Iodized salt, dairy, and seafood are more moderate sources that rarely cause problems on their own. The risk isn’t about avoiding iodine entirely. It’s about avoiding large, concentrated doses if you’re already vulnerable.
Viral Infections and Molecular Mimicry
Certain viruses, particularly those that establish lifelong latent infections, have been implicated in triggering Hashimoto’s. Epstein-Barr virus (EBV), the virus behind mononucleosis, is the most studied. Over 90% of adults carry EBV, but in genetically susceptible individuals, the virus may act as a catalyst for thyroid autoimmunity.
The primary mechanism is molecular mimicry. Proteins on the surface of EBV share a structural resemblance to thyroid-specific proteins like thyroid peroxidase and thyroglobulin. The immune system, trained to attack EBV, can mistake thyroid tissue for the virus and begin producing antibodies against it. EBV can also activate immune cells nonspecifically through what’s called the bystander effect, where inflammation in the thyroid area causes nearby immune cells to become activated even without a direct viral target. Components of EBV can also trigger innate immune sensors called toll-like receptors, releasing inflammatory cytokines that further destabilize immune balance in the thyroid.
Medications That Affect the Thyroid
Two medications are particularly well documented as triggers for thyroid autoimmunity. Amiodarone, a heart rhythm medication, has direct, dose-dependent toxic effects on thyroid cells. It causes structural damage to the thyroid, including cell death and inflammation. In people with preexisting but undiagnosed Hashimoto’s, amiodarone can unmask the condition and cause persistent hypothyroidism that doesn’t resolve even after the medication is stopped. Amiodarone also appears to increase certain immune cell populations, potentially worsening autoimmune activity in susceptible individuals.
Lithium, commonly prescribed for bipolar disorder, interferes with thyroid hormone production and release. It increases iodine content within the thyroid while blocking the final steps of hormone synthesis. In reviews covering more than 1,700 patients, the prevalence of hypothyroidism among lithium users ranged from 6% to 52%, a wide range reflecting differences in study populations and monitoring practices.
Gut Health and Intestinal Permeability
The connection between the gut and thyroid autoimmunity is an active area of investigation, but several patterns are already clear. People with Hashimoto’s tend to have higher levels of zonulin, a protein that controls the tightness of the junctions between intestinal cells. When zonulin levels rise, the gut becomes more permeable, allowing partially digested food proteins and bacterial fragments to cross into the bloodstream. This can activate the immune system in ways that spill over into autoimmune responses elsewhere in the body.
Specific gut bacteria influence immune cell behavior in ways relevant to Hashimoto’s. Segmented filamentous bacteria can stimulate the production of Th17 cells, a type of immune cell involved in autoimmune inflammation. On the other hand, a molecule produced by the bacterium Bacteroides fragilis can expand regulatory immune cells that normally keep autoimmunity in check. Shifts in the balance of these bacterial populations may influence whether the immune system stays tolerant of thyroid tissue or turns against it.
Pregnancy as a Trigger
Pregnancy involves a dramatic reshuffling of the immune system. During pregnancy, immune activity is suppressed to protect the fetus. After delivery, the immune system rebounds, sometimes overshooting into autoimmune territory. This rebound can cause postpartum thyroiditis, a temporary inflammation of the thyroid that occurs in the months after giving birth.
Most women with postpartum thyroiditis recover normal thyroid function within three to six months. But 20% to 50% go on to develop permanent hypothyroidism, particularly those who already have TPO antibodies or a history of autoimmune thyroid disease. For many women, a postpartum thyroiditis episode is the first sign that Hashimoto’s has been developing quietly in the background.
Selenium and Nutritional Factors
Selenium is a trace mineral that plays a direct role in thyroid function. The thyroid contains more selenium per gram of tissue than any other organ, and selenium-dependent enzymes help convert thyroid hormones into their active form while also protecting thyroid cells from oxidative damage. Low selenium status has been associated with higher rates of thyroid autoimmunity, and supplementation has been studied as a way to reduce TPO antibody levels.
However, supplementation isn’t universally beneficial. People with serum selenium levels at or above 122 micrograms per liter are advised not to supplement, as doing so may increase the risk of other health problems, including cancer and type 2 diabetes. Vitamin D deficiency has also been linked to Hashimoto’s in observational studies, though the exact serum levels that increase risk are not yet firmly established. Brazil nuts, seafood, and organ meats are among the richest dietary sources of selenium.
Why It All Adds Up
Hashimoto’s is rarely triggered by a single cause. The typical pattern is a genetically predisposed person who encounters one or more environmental triggers: a viral infection, a period of excess iodine intake, pregnancy, a medication, or chronic gut inflammation. Each of these factors alone might not be enough, but in combination, they can push the immune system past a tipping point. This layered causation explains why Hashimoto’s often appears in midlife, after years of accumulated exposures, though it can develop at any age. It also explains why the disease runs in families without following a simple inheritance pattern. You don’t inherit Hashimoto’s directly. You inherit a susceptibility that your environment and life circumstances may or may not activate.