What are the diagnostic findings of subacute thyroiditis?

Diagnostic Findings of Subacute Thyroiditis

Subacute thyroiditis is diagnosed by the combination of neck pain (present in 74-96% of cases), suppressed TSH with elevated thyroid hormones, markedly elevated inflammatory markers (ESR/CRP in 88% of cases), and characteristically low or absent radioactive iodine uptake on thyroid scintigraphy. 1, 2

Clinical Presentation

The classic symptom triad includes:

• Neck pain is the most consistent feature, occurring in 74-96% of patients, often radiating to the jaw or ears 1, 2
• Systemic symptoms including weakness (61%), fever (approximately 50%), and constitutional symptoms 1
• Thyrotoxic symptoms such as palpitations (50%), weight loss (42%), heat intolerance (21%), and sweating (21%) 1

Important caveat: Approximately one-third of patients are initially misdiagnosed with upper respiratory tract infection, leading to diagnostic delays averaging 8 days from first clinic visit to appropriate testing 1

Laboratory Findings

Thyroid Function Tests

• Suppressed or undetectable TSH in all patients 1
• Elevated free T4 and T3, typically about twice the upper limit of normal 1
• Characteristic T4/T3 ratio is significantly higher in subacute thyroiditis compared to Graves' disease, because T3 elevation is proportionally less pronounced 3

Inflammatory Markers

• Elevated ESR and CRP occur in 88% of patients and are essential diagnostic markers 1
• These acute phase reactants distinguish subacute thyroiditis from other causes of thyrotoxicosis 4

Imaging Studies

Thyroid Scintigraphy (Definitive Test)

• Low or absent radioactive iodine uptake is the gold standard diagnostic finding that differentiates subacute thyroiditis from Graves' disease and toxic nodular goiter 5, 6
• This test should be performed when the diagnosis is uncertain based on clinical and laboratory findings alone 6

Ultrasound Findings

• Focal or multifocal hypoechoic lesions with poorly defined, heterogeneous echogenicity 6
• These lesions can mimic suspicious thyroid nodules or malignancy on imaging 6
• Critical pitfall: Ultrasound features may appear concerning enough to prompt FNA, but these lesions resolve spontaneously with treatment of the thyroiditis 6

Diagnostic Algorithm

1. Suspect subacute thyroiditis in patients presenting with neck pain, especially with systemic symptoms and recent viral illness 1, 2

2. Order initial laboratory tests: TSH, free T4, free T3, ESR, and CRP 1

3. If TSH is suppressed with elevated thyroid hormones and elevated inflammatory markers, proceed to thyroid scintigraphy to confirm low uptake 5, 6

4. Ultrasound is optional but may show characteristic hypoechoic lesions; avoid FNA unless scintigraphy is unavailable or findings persist after thyroiditis resolution 6

Natural History and Follow-up Implications

• Subacute thyroiditis due to destructive thyroiditis resolves spontaneously and typically does not require treatment beyond symptomatic therapy with NSAIDs or beta-blockers 5
• Early transient hypothyroidism occurs in 29-37% of patients during recovery 2
• Permanent hypothyroidism develops in only 15% of patients at long-term follow-up (mean 28 years), with 25% of those who received corticosteroid therapy versus 10% who did not developing permanent hypothyroidism 2
• Recurrence is uncommon, occurring in only 4% of patients 6-21 years after initial episode 2