What is Hashimoto's Thyroiditis?
Hashimoto's thyroiditis (also called autoimmune thyroiditis or chronic lymphocytic thyroiditis) is an autoimmune disorder where the immune system attacks the thyroid gland, causing lymphocytic infiltration and progressive destruction of thyroid tissue, most commonly leading to hypothyroidism. 1, 2
Epidemiology and Demographics
- Hashimoto's thyroiditis is the most common autoimmune endocrine disease worldwide, with an annual incidence of 0.3-1.5 per 1000 people and a prevalence of 8% in the general population 1
- Women are affected 7-10 times more often than men 2
Pathophysiology
The disease develops through a complex interaction of factors:
- Genetic susceptibility combined with X-chromosome inactivation patterns, environmental triggers (iodine supply, infections, stress), and microbiome composition lead to breakdown of self-tolerance mechanisms 2
- Immune-mediated destruction occurs through thyroid infiltration by lymphocytes and antibody-mediated autoimmune response, primarily through antibodies against thyroid peroxidase (TPOAbs) 2
- Histopathologic features include lymphoplasmacytic infiltration, lymphoid follicle formation with germinal centers, parenchymal atrophy, and the presence of large follicular cells and oxyphilic (Askanazy) cells 3
Clinical Presentation
Hashimoto's thyroiditis typically follows a triphasic pattern of thyroid dysfunction: 4
- Thyrotoxicosis phase: Initial hyperthyroidism from release of preformed thyroid hormones from damaged thyroid cells 4
- Hypothyroid phase: Occurs when thyroid hormone stores are depleted 4
- Euthyroid phase: May represent either compensation by preserved thyroid tissue or eventual restoration of normal function 2, 4
Some patients may develop permanent hypothyroidism requiring lifelong treatment 4
Diagnostic Criteria
Diagnosis is based on three key elements:
- Clinical characteristics: Presence of hypothyroid symptoms, often with a painless goiter 4, 3
- Positive serum antibodies: Thyroid peroxidase antibodies (TPOAbs) and/or thyroglobulin antibodies 2, 3
- Lymphocytic infiltration on cytological examination when performed 3
Associated Risks
- Pregnancy complications: Presence of TPOAbs is associated with a 2 to 4-fold increase in risk of recurrent miscarriages and preterm birth 2
- Malignancy risk: HT is associated with 1.6 times higher risk of papillary thyroid cancer and 60 times higher risk of thyroid lymphoma compared to the general population 2
Treatment Approach
Management depends on thyroid functional status: 2, 4
- Thyrotoxicosis phase: Symptom control with beta-blockers; no antithyroid drugs needed as this is not true hyperthyroidism 4
- Euthyroid state: Periodic TSH measurements (every 6-12 months) to monitor for progression to hypothyroidism 5
- Hypothyroidism: Thyroid hormone replacement therapy with levothyroxine at doses of 1.4 to 1.8 mcg/kg/day based on lean body mass and degree of preserved thyroid function 2
Important Clinical Caveats
- Avoid unnecessary testing: Testing for thyroid autoantibodies or performing thyroid imaging in asymptomatic euthyroid patients should be avoided per current guidelines, as this can lead to overtreatment and potential harm 1
- Persistent symptoms despite euthyroidism: A substantial proportion of patients continue to experience symptoms despite achieving biochemical euthyroidism, which may be related to reduced FT3/FT4 ratio or persistently elevated antibody titers 6
- Pregnancy considerations: Women who are pregnant or planning pregnancy with elevated TSH should be treated, and those already on levothyroxine should have dose appropriateness assessed 5