What is Hashimoto's thyroiditis?

Hashimoto's Thyroiditis: Diagnosis, Clinical Features, and Management

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 often leads to progressive thyroid dysfunction. 1

Definition and Pathophysiology

Hashimoto's thyroiditis is an autoimmune disorder characterized by:

• Thyroid-specific autoantibodies (primarily thyroid peroxidase [TPO] and thyroglobulin [TG] antibodies) 1, 2
• Lymphoplasmacytic infiltration of the thyroid gland 2
• Progressive destruction of thyroid tissue leading to hypothyroidism 3
• Genetic susceptibility influenced by environmental factors and microbiome composition 3
• Women are affected 7-10 times more frequently than men 3

Clinical Presentation

Hashimoto's thyroiditis typically follows a triphasic pattern of thyroid dysfunction:

1. Initial hyperthyroid phase (thyrotoxicosis): Due to release of preformed thyroid hormone from damaged thyroid cells 4
2. Hypothyroid phase: Occurs when thyroid hormone stores are depleted 4
3. Euthyroid phase or permanent hypothyroidism: Some patients recover normal function, while others develop permanent hypothyroidism 4

Common symptoms include:

• Fatigue
• Muscle cramps
• Constipation
• Cold intolerance
• Hair loss
• Voice changes
• Weight gain
• Intellectual slowness
• Insomnia 1

Physical examination may reveal:

• Painless goiter (enlarged thyroid)
• Dry skin
• Bradycardia
• Delayed relaxation of deep tendon reflexes

Diagnosis

Diagnosis is based on:

1. Laboratory testing:

   • TSH (elevated in hypothyroidism) - most sensitive initial screening test 1
   • Free T4 (low in overt hypothyroidism) - distinguishes between subclinical and overt hypothyroidism 1
   • TPO antibodies and TG antibodies - diagnostic markers for autoimmune thyroid disease 1

2. Imaging:

   • Thyroid ultrasound showing heterogeneous echogenicity, hypoechogenicity, and increased vascularity 1

3. Fine needle aspiration (when indicated):

   • Lymphocytic infiltration on cytological examination 1

Associated Conditions

Hashimoto's thyroiditis is frequently associated with other autoimmune disorders:

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

Treatment

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

• Levothyroxine (LT4) oral therapy at doses ranging from 1.4 to 1.8 mcg/kg/day based on the degree of preserved thyroid functionality and lean body mass 1, 3

Dosing considerations:

• For patients without risk factors (not elderly, frail, or with cardiac disease), full replacement can be estimated using ideal body weight for a dose of approximately 1.6 mcg/kg/d 5
• For those older than 70 years and/or frail patients with multiple comorbidities (including cardiac disease), consider titrating up from a lower starting dose of 25-50 mg 5

Monitoring:

• TSH and Free T4 every 4-6 weeks during initial treatment 1
• Once stable, monitoring every 6-12 months 1
• More frequent monitoring (every 2-4 weeks) in pregnant women with Hashimoto's thyroiditis 1

Special Considerations

1. Pregnancy: TPO antibodies are associated with a 2-4 fold increased risk of recurrent miscarriages and preterm birth 3. Only levothyroxine is indicated during pregnancy, as T3 does not sufficiently cross the fetal blood-brain barrier 3.

2. Cancer risk: Hashimoto's thyroiditis 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 3.

3. Transient thyroiditis: Elevated TSH can be seen in the recovery phase of thyroiditis. In asymptomatic patients with FT4 that remains in the reference range, monitoring for 3-4 weeks before treating may be appropriate to determine if there is recovery to normal thyroid function 5.

Common Pitfalls to Avoid

• Attributing all symptoms to depression without screening for thyroid dysfunction
• Inadequate thyroid hormone replacement
• Overlooking subclinical hypothyroidism
• Ignoring thyroid antibodies
• Development of a low TSH on therapy suggests overtreatment or recovery of thyroid function and dose should be reduced or discontinued with close follow-up 5, 1