Etiology of Thyroiditis
Thyroiditis is primarily caused by autoimmune processes, with Hashimoto's thyroiditis being the most common form, followed by postpartum thyroiditis, subacute thyroiditis, and drug-induced thyroiditis. 1
Autoimmune Causes
- Hashimoto's thyroiditis is an autoimmune disorder characterized by thyroid-specific autoantibodies (particularly thyroid peroxidase antibodies), lymphoplasmacytic infiltration of the thyroid gland, and often gradual progression to hypothyroidism 2
- Graves' disease can cause inflammatory changes in the thyroid gland, leading to thyroiditis as part of its autoimmune process 3
- Postpartum thyroiditis occurs within one year of delivery, miscarriage, or medical abortion and is believed to result from a general decline in T suppressor lymphocyte function after delivery 1, 4
- Silent (painless) thyroiditis is an autoimmune disease with more acute onset than Hashimoto's, characterized by a triphasic pattern of thyroid dysfunction (hyperthyroidism, hypothyroidism, recovery) 4
Infectious and Inflammatory Causes
- Subacute (granulomatous) thyroiditis often follows viral upper respiratory infections, with the thyroid becoming an "innocent bystander" in an immune-mediated antiviral attack 4, 5
- Sarcoidosis can involve the thyroid gland as part of a multisystem granulomatous disease 3
Medication-Induced Causes
- Amiodarone can cause both type I (iodine-induced) and type II (destructive) thyroiditis 3, 1
- Other medications that can induce thyroiditis include:
Environmental and Other Causes
- Excessive iodine intake can trigger thyroiditis in susceptible individuals 3
- Radiation exposure to the head and neck area can cause thyroiditis 6
- Physical trauma to the thyroid gland can result in inflammatory thyroiditis 6
Pathophysiological Mechanisms
- In autoimmune thyroiditis, both cellular and humoral immunity play key roles, with T and B cell inflammatory infiltration commonly found 2
- Histopathologic features include lymphoplasmacytic infiltration, lymphoid follicle formation with germinal centers, and parenchymal atrophy 2
- Most forms of thyroiditis follow a triphasic pattern of thyroid dysfunction:
- Initial hyperthyroidism (thyrotoxicosis) due to release of preformed thyroid hormone from damaged thyroid cells
- Subsequent hypothyroidism when thyroid hormone stores are depleted
- Eventual restoration of normal thyroid function in many cases, though some patients develop permanent hypothyroidism 1
Risk Factors for Thyroid Dysfunction
- Female sex increases risk for both hypothyroidism and hyperthyroidism 7
- Advancing age is associated with increased risk of thyroid dysfunction 7
- Personal or family history of thyroid disease 7
- Type 1 diabetes increases risk for hypothyroidism 7
- Down syndrome is associated with increased risk of hypothyroidism 7
- Previous hyperthyroidism may lead to iatrogenic thyroid dysfunction 7
Diagnostic Considerations
- Doppler ultrasound can help distinguish between thyrotoxicosis due to overactive thyroid (increased blood flow) versus destructive thyroiditis (decreased blood flow) 3
- Patients with positive thyroid antibodies have a 4.3% per year risk of developing overt hypothyroidism versus 2.6% per year in antibody-negative individuals 3
- High TPO antibodies are the strongest predictor of progression to hypothyroidism 3
Common Pitfalls in Diagnosis and Management
- Failing to recognize the triphasic pattern of thyroid dysfunction in many forms of thyroiditis can lead to inappropriate treatment 1
- In postpartum thyroiditis, symptoms may be mistaken for normal postpartum changes or postpartum depression 7
- When treating both adrenal insufficiency and hypothyroidism, steroids should always be started prior to thyroid hormone to avoid precipitating an adrenal crisis 7
- Regular monitoring of thyroid function is essential in all forms of thyroiditis, with follow-up every 6-12 months 3, 1