How do you differentiate thyroiditis from hypothyroidism?

Differentiating Thyroiditis from Hypothyroidism

Thyroiditis is an inflammatory condition of the thyroid gland that often progresses through distinct phases (hyperthyroidism, hypothyroidism, and recovery), while hypothyroidism is a clinical state of thyroid hormone deficiency that can result from various causes including thyroiditis. 1

Key Differences

Etiology and Pathophysiology

• Thyroiditis is inflammation of the thyroid gland that can be caused by autoimmune disorders, viral infections, radiation, medications, or trauma 1, 2
• Hypothyroidism is a clinical state of thyroid hormone deficiency that can result from:
  • Primary causes: thyroid gland dysfunction (including Hashimoto's thyroiditis, post-surgical, post-radioiodine therapy) 3
  • Secondary/central causes: pituitary or hypothalamic dysfunction 4

Clinical Course

• Thyroiditis often follows a triphasic pattern:
  • Initial hyperthyroid phase due to release of preformed thyroid hormone from damaged thyroid cells 1
  • Subsequent hypothyroid phase when thyroid hormone stores are depleted 1
  • Recovery phase with restoration of normal thyroid function (though some patients develop permanent hypothyroidism) 1, 5
• Hypothyroidism typically presents as a persistent state of thyroid hormone deficiency requiring ongoing replacement therapy 3

Laboratory Findings

• Thyroiditis:
  • Acute/subacute phase: Low TSH with elevated free T4/T3 (thyrotoxic phase) 3
  • Later phase: Elevated TSH with low free T4 (hypothyroid phase) 3
  • May have elevated inflammatory markers (ESR, CRP) in subacute thyroiditis 5
  • Thyroid antibodies may be present in autoimmune thyroiditis 1
• Hypothyroidism:
  • Primary: Elevated TSH with low free T4 3
  • Central/secondary: Low or normal TSH with low free T4 4

Radioactive Iodine Uptake

• Thyroiditis (during thyrotoxic phase): Low radioactive iodine uptake due to inflammation rather than increased hormone production 5
• Hypothyroidism: All causes show decreased radioactive iodine uptake, making this test not useful for differentiation 3

Clinical Features

• Thyroiditis:
  • Subacute thyroiditis: Often presents with neck pain and tenderness 1
  • Postpartum thyroiditis: Occurs within one year of childbirth, miscarriage, or abortion 1
  • May have a palpable goiter 1
• Hypothyroidism:
  • Presents with fatigue, cold intolerance, weight gain, constipation, dry skin 3
  • May have a non-tender goiter in Hashimoto's thyroiditis 1

Diagnostic Approach

Laboratory Testing

• TSH and free T4 are essential initial tests 3
• Consider thyroid antibodies (TPO, TRAb) to identify autoimmune etiology 3
• In suspected thyroiditis, monitor thyroid function tests every 2-3 weeks initially to observe the typical pattern 3

Imaging

• Imaging generally not helpful in differentiating causes of hypothyroidism 3
• Doppler ultrasound may help distinguish hyperthyroidism from thyroiditis during thyrotoxic phase:
  • Increased blood flow in Graves' disease
  • Decreased blood flow in thyroiditis 3

Management Considerations

Thyroiditis

• Subacute thyroiditis:
  • Symptomatic treatment with NSAIDs or corticosteroids for pain 1
  • Beta-blockers for hyperthyroid symptoms 3
  • Monitor for transition to hypothyroid phase 3
  • Most cases resolve spontaneously within months 5
• Postpartum thyroiditis:
  • Monitor thyroid function through phases 3
  • Consider levothyroxine if TSH >10 mIU/L or if symptomatic with TSH 4-10 mIU/L 5

Hypothyroidism

• Primary hypothyroidism: Levothyroxine replacement (1.6 mcg/kg/day in young healthy patients; 25-50 mcg in elderly or those with cardiac disease) 3
• Central hypothyroidism: Levothyroxine dosed based on free T4 levels (not TSH), aiming for upper half of normal range 4
• Monitor TSH and free T4 6-8 weeks after initiation or dose changes 3

Clinical Pearls

• A patient with low TSH and high free T4 could be experiencing either hyperthyroidism or the thyrotoxic phase of thyroiditis - clinical context and additional testing help differentiate 6
• Thyroiditis is often self-limiting while hypothyroidism typically requires lifelong hormone replacement 1
• Both conditions can present with fatigue, making clinical distinction challenging without laboratory testing 6
• Always consider medication-induced thyroiditis (amiodarone, immune checkpoint inhibitors, lithium) in the differential diagnosis 1