What is the most common cause of hyperthyroidism in a 66-year-old woman?

Most Common Cause of Hyperthyroidism in a 66-Year-Old Woman

Toxic multinodular goiter is the most common cause of hyperthyroidism in a 66-year-old woman. 1

Epidemiology and Causes of Hyperthyroidism in Older Adults

The etiology of hyperthyroidism varies significantly with age, with different patterns emerging in older populations compared to younger individuals:

• Toxic multinodular goiter: 43.1% of hyperthyroidism cases in patients over 55 years 1
• Graves' disease: 21.4% of cases in older adults 1
• Iatrogenic thyrotoxicosis: 16.0% of cases 1
• Toxic adenoma: 11.8% of cases 1

This contrasts with younger populations where Graves' disease predominates as the primary cause of hyperthyroidism.

Pathophysiology of Toxic Multinodular Goiter

Toxic multinodular goiter develops gradually over many years, typically becoming clinically significant in the fifth and sixth decades of life, with higher prevalence in women 2. The condition involves:

• Multiple autonomously functioning thyroid nodules
• Gradual enlargement of the thyroid gland
• Functional independence from TSH regulation
• Possible substernal extension in advanced cases

Clinical Presentation in Older Women

The presentation of hyperthyroidism in older women often differs from classic hyperthyroid symptoms seen in younger patients:

• Cardiovascular manifestations: Often the first and sometimes only symptoms (cardiac failure, arrhythmias) 3
• Atypical presentations: Depression, low-grade fever, fatigue, or nausea 3
• Compressive symptoms: Dyspnea, dysphagia, or hoarseness if goiter is large 4
• Subclinical presentation: Up to 46.6% of older patients may have subclinical hyperthyroidism 1

Diagnostic Approach

For suspected hyperthyroidism in a 66-year-old woman:

1. Initial laboratory testing: TSH is the primary screening test 2

   • Suppressed TSH with elevated free T4/T3 indicates overt hyperthyroidism
   • Suppressed TSH with normal free T4/T3 suggests subclinical hyperthyroidism

2. Follow-up testing: Multiple tests should be done over 3-6 months to confirm abnormal findings 2

3. Imaging studies:

   • Thyroid ultrasound: First-line imaging to confirm multinodular goiter 2
   • Radionuclide uptake scan: Helps distinguish between causes of hyperthyroidism 2
     • Toxic multinodular goiter shows multiple areas of increased uptake
     • Graves' disease shows diffuse increased uptake
     • Thyroiditis shows decreased uptake

Treatment Considerations

Treatment options for toxic multinodular goiter in older women include:

• Radioactive iodine ablation: Most widely used treatment in the United States 5
• Antithyroid medications: Methimazole preferred over propylthiouracil in older adults 5
• Surgery: Reserved for large goiters causing compressive symptoms or when other treatments fail 2

Important Clinical Pitfalls

1. Misdiagnosis risk: Atypical presentation in older women may lead to delayed diagnosis
2. Cardiovascular complications: Untreated hyperthyroidism increases risk of atrial fibrillation and heart failure
3. Medication interactions: Amiodarone and other iodine-containing drugs can precipitate or worsen hyperthyroidism 2
4. Intrathoracic extension: Large multinodular goiters may extend into the thorax, requiring specialized surgical approach 4
5. Treatment monitoring: Older patients may require closer monitoring due to increased risk of treatment complications

In summary, when evaluating hyperthyroidism in a 66-year-old woman, toxic multinodular goiter should be the primary diagnostic consideration, followed by Graves' disease, iatrogenic causes, and toxic adenoma.