Causes of Thyroiditis
Thyroiditis is most commonly caused by autoimmune conditions (Hashimoto's thyroiditis), followed by viral infections, medications, radiation, and trauma, with each type presenting distinct clinical patterns and requiring specific management approaches. 1, 2
Autoimmune Causes
- Hashimoto's thyroiditis (chronic lymphocytic thyroiditis) - the most common form of thyroiditis, characterized by elevated thyroid peroxidase (TPO) antibodies and gradual thyroid destruction 2, 3
- Graves' disease - while primarily causing hyperthyroidism, can also cause inflammatory changes in the thyroid gland 4
- Postpartum thyroiditis - occurs within one year of delivery, miscarriage, or medical abortion, affecting 5-10% of postpartum women 2
Infectious Causes
- Acute suppurative (bacterial) thyroiditis - typically caused by bacterial infection, often Staphylococcus or Streptococcus species 3
- Subacute granulomatous thyroiditis (de Quervain's thyroiditis) - often follows upper respiratory viral infections and presents with painful thyroid and systemic symptoms 2, 5
- Viral infections - including mumps, coxsackievirus, adenovirus, and other respiratory viruses 2
Medication-Induced Causes
- Amiodarone - can cause both type I (iodine-induced) and type II (destructive) thyroiditis 4, 2
- Immune checkpoint inhibitors - increasingly recognized cause of thyroiditis in cancer patients 2
- Interferon-alfa and interleukin-2 - can trigger autoimmune thyroid inflammation 1, 2
- Lithium - disrupts thyroid hormone synthesis and release 2
- Tyrosine kinase inhibitors - can cause destructive thyroiditis 2
Environmental/External Causes
- Radiation exposure - including therapeutic radiation for cancer treatment 1
- Trauma to the thyroid gland - including surgical manipulation 1
- Excessive iodine intake - can trigger thyroiditis in susceptible individuals 4
Rare Causes
- Riedel's thyroiditis (invasive fibrous thyroiditis) - extremely rare condition characterized by fibrosis of the thyroid that can extend into surrounding tissues 3, 6
- Sarcoidosis - can involve the thyroid gland as part of multisystem granulomatous disease 4
Clinical Patterns and Disease Course
- Most forms of thyroiditis follow a triphasic pattern: initial hyperthyroidism (thyrotoxicosis) due to release of preformed thyroid hormone, followed by hypothyroidism as stores are depleted, and eventual return to normal function in many cases 2
- Some patients, particularly those with Hashimoto's thyroiditis, develop permanent hypothyroidism requiring lifelong thyroid hormone replacement 2, 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 7
Diagnostic Considerations
- Diagnosis is based on clinical context, presence/absence of pain, laboratory findings (including thyroid function tests and autoantibodies), and reduced radioactive iodine uptake in most inflammatory conditions 1
- Doppler ultrasound can help distinguish between thyrotoxicosis due to overactive thyroid (increased blood flow) versus destructive thyroiditis (decreased blood flow) 4
- High TPO antibodies are the strongest predictor of progression to hypothyroidism 7
Management Approach
- Treatment is directed at symptom relief and restoration of normal thyroid function 1
- Pain management with NSAIDs or corticosteroids for painful forms of thyroiditis 2
- Beta-blockers for adrenergic symptoms during the hyperthyroid phase 2
- Levothyroxine for hypothyroidism, either temporary or permanent depending on the type of thyroiditis 2
- Regular monitoring of thyroid function is essential, with follow-up every 6-12 months 7
Understanding the specific cause of thyroiditis is crucial for appropriate management and predicting disease course, with particular attention to the risk of permanent hypothyroidism in autoimmune forms.