How do we diagnose thyroiditis as the cause of hyperthyroidism in an adult patient?

Identifying Thyroiditis as the Cause of Hyperthyroidism

The key to diagnosing thyroiditis-induced hyperthyroidism is demonstrating low or absent radioiodine uptake on thyroid scintigraphy, which distinguishes destructive thyroiditis from hyperfunctioning causes like Graves disease or toxic nodular goiter. 1

Diagnostic Algorithm

Step 1: Confirm Thyrotoxicosis Biochemically

• Measure TSH (suppressed/low) with elevated free T4 or total T3 1, 2
• This confirms thyrotoxicosis but does not distinguish the underlying cause 3

Step 2: Distinguish Thyroiditis from Hyperthyroidism Using Radioiodine Uptake

• Radioiodine uptake scan (I-123 or Tc-99m pertechnetate) is the preferred test to differentiate thyroiditis from true hyperthyroidism 1, 4
• Thyroiditis shows LOW or ABSENT uptake (destructive process releasing preformed hormone) 1, 5
• Graves disease/toxic nodular goiter show HIGH uptake (overactive thyroid hormone synthesis) 1, 4
• Radionuclide uptake directly measures thyroid activity rather than inferring it, making it superior to other modalities 1

Step 3: Measure TSH Receptor Antibodies

• TSH receptor antibodies (TRAb) or thyroid stimulating immunoglobulin (TSI) help rule out Graves disease 1, 2
• Positive antibodies indicate Graves disease (not thyroiditis) 2, 6
• Negative antibodies support thyroiditis when combined with low uptake 1

Step 4: Consider Doppler Ultrasound as Alternative

• Doppler ultrasound can differentiate thyroiditis (decreased blood flow) from Graves/toxic adenoma (increased blood flow) 1
• Sensitivity and specificity are comparable to radionuclide studies (95% and 90% vs 90% and 100%) 1
• However, radionuclide uptake remains preferred because it directly measures thyroid activity 1
• Use Doppler when recent iodinated contrast precludes radioiodine scanning 1

Step 5: Assess Clinical Context

• Thyroiditis is typically painless and self-limited, with thyrotoxic phase lasting approximately 1 month 1, 5
• Subacute (granulomatous) thyroiditis presents with anterior neck pain (3% of hyperthyroidism cases) 2, 5
• Drug-induced thyroiditis (amiodarone, immune checkpoint inhibitors, tyrosine kinase inhibitors) requires medication history 1, 2, 5
• Thyroiditis most commonly follows a triphasic pattern: thyrotoxicosis → hypothyroidism → recovery 5

Key Distinguishing Features

Thyroiditis Pattern:

• Low/absent radioiodine uptake 1, 4
• Negative TSH receptor antibodies 1
• Decreased thyroid blood flow on Doppler 1
• Self-limited course (1-2 months) 1, 5
• Often progresses to permanent hypothyroidism 1, 5

Graves Disease/Toxic Nodular Goiter Pattern:

• High radioiodine uptake 1, 4
• Positive TSH receptor antibodies (Graves) 2, 6
• Increased thyroid blood flow on Doppler 1
• Persistent hyperthyroidism requiring definitive treatment 2, 3

Common Pitfalls to Avoid

• Do not rely solely on clinical presentation or thyroid antibodies (TPO), as these do not distinguish thyroiditis from other causes of thyrotoxicosis 1
• Do not skip radioiodine uptake when the diagnosis is unclear, as this is the definitive test to separate destructive from hyperfunctioning causes 1, 4
• Recognize that most thyroiditis is asymptomatic (painless thyroiditis) and detected only through routine monitoring 1
• Remember that thyroiditis requires only conservative management (beta blockers for symptoms), not antithyroid drugs, radioiodine, or surgery 1, 5
• Monitor for progression to hypothyroidism with repeat thyroid function tests every 2-3 weeks during the thyrotoxic phase 1