What is Hashimoto's thyroiditis?

Hashimoto's Thyroiditis

Hashimoto's thyroiditis is the most common autoimmune thyroid disorder and the leading cause of hypothyroidism in industrialized nations, characterized by chronic autoimmune destruction of the thyroid gland that leads to progressive thyroid dysfunction. 1

Definition and Pathophysiology

Hashimoto's thyroiditis (HT) is characterized by:

• Chronic autoimmune destruction of the thyroid gland mediated by both humoral and cellular immunity 2
• Lymphoplasmacytic infiltration of the thyroid gland, particularly T cells
• Formation of lymphoid follicles with germinal centers
• Progressive parenchymal atrophy and fibrosis 3
• Presence of thyroid-specific autoantibodies, primarily:
  • Thyroid peroxidase antibodies (TPO-Ab)
  • Thyroglobulin antibodies (TG-Ab) 1

Epidemiology

• Most common autoimmune thyroid disorder worldwide 4
• Leading cause of hypothyroidism in iodine-sufficient regions 1
• More common in women than men
• Often develops in middle age, but can occur at any age
• Frequently associated with other autoimmune conditions (10-23% of patients) 1

Etiology

Hashimoto's thyroiditis results from a complex interaction of:

1. Genetic factors: Strong HLA associations, particularly with HLA-DR/DQ genes 1
2. Environmental triggers:
   • Excessive iodine consumption in susceptible individuals
   • Possible roles for infections, medications, and environmental toxins 1
3. Epigenetic influences: DNA methylation and miRNA regulation 2

Clinical Presentation

Patients with Hashimoto's thyroiditis may present with:

• Symptoms of hypothyroidism:
  • Fatigue
  • Muscle cramps
  • Constipation
  • Cold intolerance
  • Hair loss
  • Voice changes (hoarseness)
  • Weight gain
  • Intellectual slowness
  • Insomnia 1
• Physical findings:
  • Goiter (painless, firm, diffusely enlarged thyroid)
  • Dry skin
  • Bradycardia
  • Delayed relaxation of deep tendon reflexes
  • Periorbital edema 1

Diagnosis

The diagnosis of Hashimoto's thyroiditis is based on:

1. Laboratory testing:

   • Elevated TSH (most sensitive initial screening test)
   • Low or normal Free T4 (to distinguish between subclinical and overt hypothyroidism)
   • Positive thyroid autoantibodies:
     • TPO antibodies (most sensitive marker)
     • Thyroglobulin antibodies 1

2. Imaging (when needed):

   • Ultrasound may show heterogeneous echotexture, hypoechogenicity, and increased vascularity

3. Fine needle aspiration (rarely needed):

   • Shows lymphocytic infiltration on cytological examination 3

Management

The standard treatment for hypothyroidism resulting from Hashimoto's thyroiditis is:

• Levothyroxine (LT4) oral therapy:

  • Dosing: 1.4 to 1.8 mcg/kg/day based on degree of preserved thyroid function and lean body mass
  • Goal: Normalize TSH levels (typically 0.5-4.0 mIU/L) 1
  • Monitoring: TSH and Free T4 every 6-12 months once stable 1

• Nutritional considerations:

  • Several studies have shown benefits of vitamin D and selenium supplementation
  • Anti-inflammatory diet rich in vitamins and minerals may have protective effects 5
  • Address common deficiencies (vitamin D, selenium, magnesium, iron, vitamin B12) 5

Associated Conditions

Hashimoto's thyroiditis is frequently associated with:

• Other autoimmune disorders:

  • Systemic lupus erythematosus (2.8-3% of HT patients)
  • Sjögren syndrome (2.8-7%)
  • Rheumatoid arthritis (2-4%)
  • Inflammatory bowel disease (2-11.4%)
  • Type 1 diabetes mellitus
  • Celiac disease
  • Addison's disease
  • Vitiligo
  • Pernicious anemia 1

• Possible increased risk of papillary thyroid cancer (relationship still debated) 4

Complications

If left untreated, Hashimoto's thyroiditis can lead to:

• Progressive hypothyroidism
• Myxedema (severe hypothyroidism with bradycardia, hypothermia, and altered mental status) 6
• Goiter
• Decreased quality of life
• Metabolic disorders even in euthyroid patients 5

Monitoring

• During treatment initiation: Monitor every 2-4 weeks until stable
• Once stable: Monitor TSH and Free T4 every 6-12 months
• More frequent monitoring (every 2-4 weeks) in pregnant women 1

Prognosis

• Most patients require lifelong thyroid hormone replacement
• With proper treatment, patients can lead normal lives with excellent prognosis
• Some patients may experience persistent symptoms despite normalized thyroid function tests, suggesting additional mechanisms beyond hypothyroidism 7