An inflamed thyroid, called thyroiditis, has several distinct causes ranging from an immune system attack on the gland to viral infections, certain medications, and rarely, bacterial infections. The most common cause by far is autoimmune, where your body’s own immune cells gradually damage thyroid tissue. Other forms can flare up suddenly after a respiratory virus or as a side effect of specific drugs. Understanding which type you’re dealing with matters because the symptoms, pain level, and outlook differ significantly.
Autoimmune Disease: The Most Common Cause
The majority of thyroid inflammation cases trace back to the immune system mistakenly targeting the thyroid gland. In Hashimoto’s thyroiditis, a specific type of white blood cell called a T cell launches an attack on thyroid tissue, triggering the production of antibodies against proteins on thyroid cells. These antibodies, particularly one called anti-TPO (the most specific marker for autoimmune thyroiditis), destroy the small hormone-producing structures in the gland called follicular cells. Over time, this ongoing destruction reduces the gland’s ability to make thyroid hormones, leading to hypothyroidism.
Hashimoto’s is painless. You won’t feel the inflammation happening. Instead, you’ll notice the downstream effects: fatigue, weight gain, cold sensitivity, and brain fog as hormone levels drop. The gland often enlarges gradually, forming a goiter that may be noticeable in the neck. Because the damage accumulates slowly over months or years, many people don’t realize anything is wrong until blood tests reveal abnormal thyroid hormone levels.
Viral Infections and Subacute Thyroiditis
Subacute thyroiditis, sometimes called de Quervain thyroiditis, typically develops two to eight weeks after a viral upper respiratory infection. It has been linked to coxsackievirus, echovirus, mumps, measles, influenza, and SARS-CoV-2. The condition follows a seasonal pattern that mirrors the peak of common viral infections.
Unlike autoimmune thyroiditis, this form hurts. Pain is the dominant symptom, reported by 96% of patients. It centers over the thyroid gland in the front of the neck and can radiate to the jaw, upper chest, and throat. Turning your head, coughing, or swallowing can make it worse. The gland feels slightly enlarged, warm, and tender when touched. A hallmark lab finding is a sharply elevated sedimentation rate (a marker of inflammation in the blood), often reaching 60 to 100 or higher, which helps distinguish it from painless forms.
Postpartum Thyroiditis
Roughly 5% to 10% of women develop thyroid inflammation within the first year after giving birth, with some studies reporting rates as high as 16% in certain populations. This is an autoimmune process, likely triggered by the immune system “rebounding” after the natural immune suppression of pregnancy.
Postpartum thyroiditis usually appears one to four months after delivery. It’s painless and often mistaken for normal postpartum fatigue or mood changes, which makes it easy to miss. The initial phase involves too much thyroid hormone being released from the damaged gland, causing anxiety, a racing heart, and insomnia. This is followed by a hypothyroid phase around four to eight months postpartum, bringing low energy, weight gain, cold intolerance, and depression. About 80% of women recover full thyroid function within 12 to 18 months.
How Thyroiditis Moves Through Phases
Most forms of transient thyroiditis, whether triggered by a virus, pregnancy, or medications, follow a predictable three-phase pattern. First comes a thyrotoxic phase lasting one to three months, where stored thyroid hormones flood the bloodstream as inflamed cells break open. Symptoms during this phase include anxiety, insomnia, fast heart rate, weight loss, and irritability.
Next comes a hypothyroid phase, typically one to three months after the first, lasting up to 9 to 12 months. Your thyroid is now depleted and can’t produce enough hormones, so you feel tired, cold, and sluggish. Finally, the gland recovers. Around 80% of people with painless or postpartum thyroiditis and about 95% of those with subacute thyroiditis return to normal thyroid function within 12 to 18 months of symptom onset.
Medications That Inflame the Thyroid
Several prescription drugs can trigger thyroid inflammation as a side effect. The best-known culprits are amiodarone (a heart rhythm medication), lithium (used for bipolar disorder), interferon alfa, and interleukin-2. These typically cause painless thyroiditis that follows the same phase pattern described above.
A newer and increasingly common cause is immune checkpoint inhibitors, a class of cancer drugs that includes pembrolizumab, nivolumab, and ipilimumab. These medications work by releasing the brakes on the immune system so it can fight tumors more aggressively. The trade-off is that the newly unleashed immune cells sometimes attack healthy tissue, and the thyroid is one of the most frequently affected organs. Thyroid dysfunction is among the most common endocrine side effects of these drugs. If you’re receiving cancer immunotherapy, your care team will typically monitor thyroid levels regularly for this reason.
Bacterial Infection: Rare but Serious
Acute infectious thyroiditis, caused by bacteria directly invading the gland, is uncommon. The thyroid is naturally resistant to infection due to its rich blood supply, high iodine content, and protective capsule. When it does occur, it tends to affect people with weakened immune systems.
The symptoms resemble a classic infection: neck pain (89% of cases), fever (82%), difficulty swallowing (46%), and a red, hot, swollen area over the thyroid. White blood cell counts are elevated in about 80% of bacterial cases. The most common bacteria involved are Staphylococcus aureus and various Streptococcus species, though in immunocompromised individuals, unusual organisms like fungi (Candida, Aspergillus) or tuberculosis bacteria can be responsible. This form requires prompt treatment, unlike most other types of thyroiditis that resolve on their own.
Riedel Thyroiditis: A Fibrous Disorder
The rarest form of thyroid inflammation is Riedel thyroiditis, where the gland is gradually replaced by dense, scar-like fibrous tissue. Rather than a straightforward immune attack, this is now understood to be part of a broader category of fibroinflammatory disease linked to elevated levels of a specific antibody called IgG4. The fibrous tissue can extend beyond the thyroid into surrounding neck structures, making the gland feel rock-hard and sometimes causing compression of the windpipe or esophagus. Because of its hardness and fixed nature, it can initially be mistaken for thyroid cancer on physical exam. Treatment often involves corticosteroids, and in resistant cases, other immune-suppressing therapies have been used successfully.
Risk Factors That Increase Susceptibility
Several factors raise your chances of developing thyroid inflammation. A family history of autoimmune disease is the strongest predictor for Hashimoto’s and other autoimmune forms. Women are affected far more often than men across nearly all types of thyroiditis. Having one autoimmune condition, such as type 1 diabetes or rheumatoid arthritis, increases the likelihood of developing another, including autoimmune thyroiditis.
For subacute thyroiditis, recent viral illness is the key risk factor, particularly during fall and winter when respiratory viruses peak. For postpartum thyroiditis, women who test positive for anti-TPO antibodies during pregnancy have a significantly higher risk of developing the condition after delivery. And for drug-induced forms, simply being on one of the known triggering medications puts you in a higher-risk category, particularly if you’re receiving combination immunotherapy for cancer.