Can you summarize Hashimoto thyroiditis, the autoimmune cause of primary hypothyroidism?

Hashimoto's Thyroiditis Overview

Hashimoto's thyroiditis is the most common cause of hypothyroidism in iodine-sufficient areas like the United States, representing a chronic autoimmune destruction of the thyroid gland that predominantly affects women (5-10 times more than men) and increases in frequency with age. 1

Pathophysiology

The disease results from an inherited defect in immune surveillance leading to loss of self-tolerance against thyroid antigens 1. The hallmark pathologic features include:

• Lymphocytic infiltration of the thyroid gland, particularly sensitized T lymphocytes (CD4+ T cells) 1, 2
• Progressive destruction of thyroid follicles leading to atrophy and fibrosis 2
• Imbalance in Th17/Treg cell ratio driving the autoimmune process 3
• Both cellular (T cell-mediated) and humoral (B cell/antibody-mediated) immune responses 3

The disease develops from a combination of genetic susceptibility and environmental triggers, with gut microbiota and X-chromosome inactivation patterns also playing contributory roles 4, 5.

Epidemiology

Based on population studies 1:

• Subclinical hypothyroidism: 4.3-8.5% of the population
• Overt hypothyroidism: 0.3-0.4% of the population
• Gender distribution: 5% of women vs. 1% of men have both positive antibodies and elevated TSH
• Age-related increase: Prevalence rises significantly with advancing age

Diagnostic Criteria

Diagnosis requires three key elements 1, 4:

1. Thyroid function tests:

   • Subclinical: TSH above upper reference limit (>4.12 mIU/L) with normal free T4
   • Overt: TSH usually >10 mIU/L with subnormal free T4
   • Note: TSH must be stable for weeks, not during acute illness or recent medication changes 6

2. Thyroid autoantibodies 1:

   • Anti-thyroid peroxidase antibodies (TPOAb): Present in ~75% of patients, most clinically significant
   • Anti-thyroglobulin antibodies (TgAb): Present in many patients
   • TPOAb positivity predicts progression to overt hypothyroidism at 4.3% per year (vs. 2.6% without antibodies)

3. Thyroid ultrasound 4:

   • Shows heterogeneous echotexture
   • May reveal diffuse goiter or atrophic gland
   • Helps identify nodular variant requiring fine-needle aspiration

Critical diagnostic pitfall: Exclude other causes of elevated TSH before diagnosing Hashimoto's, including recent levothyroxine dose adjustments, recovery from severe illness, untreated adrenal insufficiency, and heterophilic antibodies causing false-positive TSH 6.

Clinical Presentation Spectrum

The disease progresses through distinct phases 4:

A. Hashitoxicosis (early phase):

• Transient hyperthyroid symptoms from release of stored hormones during thyrocyte destruction
• Managed with β-blockers for symptom control, NOT antithyroid drugs

B. Euthyroid phase:

• Preserved thyroid tissue compensates for destroyed cells
• Requires periodic TSH monitoring (typically every 6-12 months) to detect progression

C. Hypothyroid phase:

• Insufficient hormone production from destroyed gland
• Requires levothyroxine replacement therapy

Associated Conditions and Complications

Autoimmune clustering 1, 2:

• 20-30% of patients have concurrent autoimmune disorders
• More common in patients and their families
• Screen for celiac disease, type 1 diabetes, adrenal insufficiency, vitiligo

Malignancy risk 4:

• 1.6-fold increased risk of papillary thyroid cancer
• 60-fold increased risk of thyroid lymphoma
• Maintain vigilance for rapidly enlarging goiter or new dominant nodules

Pregnancy complications 4:

• TPOAb presence associated with 2-4 fold increase in recurrent miscarriages and preterm birth
• Requires aggressive TSH control (<2.5 mIU/L in first trimester)

Metabolic consequences 7:

• Excess body weight even in euthyroid state
• Dyslipidemia (resolves with treatment)
• Reduced quality of life

Treatment Approach

Primary treatment is levothyroxine replacement when hypothyroidism develops 4, 2:

• Dosing: 1.4-1.8 mcg/kg/day based on lean body mass and degree of preserved thyroid function
• Goal TSH: Generally 0.5-2.5 mIU/L (lower in pregnancy: <2.5 mIU/L first trimester)
• Monitoring: TSH every 6-8 weeks after dose changes, then every 6-12 months when stable

Important treatment considerations:

• Use only levothyroxine (T4) in pregnancy - T3 does not adequately cross the fetal blood-brain barrier 4
• Insufficient evidence for routine T4/T3 combination therapy outside pregnancy 4
• Do NOT use selenium as treatment or prevention per guideline recommendations 1
• Avoid dietary supplements marketed as "thyroid support" 1

Adjunctive therapies with modest evidence 7, 8:

• Vitamin D supplementation if deficient
• Selenium supplementation (200 mcg/day) may reduce antibody titers but doesn't change clinical outcomes
• Screen and correct deficiencies in iron, magnesium, vitamin B12

For persistent symptoms despite biochemical euthyroidism 8:

• Verify medication adherence and proper levothyroxine administration (empty stomach, 30-60 minutes before food)
• Check for malabsorption, drug interactions, or pharmacokinetic issues
• Screen for coexisting conditions: depression, anxiety, other autoimmune diseases, vitamin deficiencies
• Consider FT3/FT4 ratio assessment
• Total thyroidectomy reserved only for refractory cases with severe quality of life impairment after exhausting medical options

No specific diet is universally recommended 7, though an anti-inflammatory diet rich in vitamins and minerals may be beneficial. Insufficient evidence supports routine gluten-free diet for all patients with Hashimoto's 7.

Monitoring Strategy

For euthyroid patients with positive antibodies 1:

• TSH measurement every 6-12 months
• Higher surveillance frequency if TPOAb-positive (4.3% annual progression risk)
• Measure TPOAb in patients with diffuse firm goiter to distinguish from multinodular goiter

For treated hypothyroid patients:

• TSH every 6-8 weeks after dose adjustments
• TSH every 6-12 months once stable
• More frequent monitoring during pregnancy, with dose adjustments, or if symptoms develop