What is the most likely diagnosis and recommended next step for a patient whose static Tc‑99m pertechnetate thyroid scan performed 20 minutes after 2 mCi injection shows a normally sized thyroid with uniformly reduced uptake in both lobes and normal salivary gland uptake?

Diagnosis and Management of Reduced Thyroid Uptake on Tc-99m Pertechnetate Scan

Most Likely Diagnosis

The most likely diagnosis is thyroiditis (subacute, painless/lymphocytic, or drug-induced), which presents as a destructive thyroid process causing reduced radiotracer uptake despite thyrotoxicosis. 1

The key diagnostic feature here is uniformly reduced uptake in both thyroid lobes with normal salivary gland uptake, which indicates the thyroid gland is not actively trapping pertechnetate despite being structurally normal in size. This pattern is pathognomonic for a destructive thyroid process rather than hyperfunctioning states like Graves disease or toxic nodular goiter. 1

Differential Diagnosis Based on Uptake Pattern

The reduced bilateral thyroid uptake with normal salivary uptake narrows the differential to:

• Thyroiditis (most common): Subacute (painful), painless/lymphocytic, postpartum, or drug-induced (amiodarone type II, immune checkpoint inhibitors) 2, 1
• Hypothyroidism: All causes show decreased uptake, but this is typically a chronic presentation 2
• Factitious thyrotoxicosis: Exogenous thyroid hormone ingestion suppresses uptake 2
• Iodine excess: Recent iodinated contrast or amiodarone (type I) can reduce uptake 2

The normal salivary gland uptake is critical because it confirms the Tc-99m pertechnetate is functioning properly and being trapped by sodium-iodide symporter in other tissues, ruling out technical issues with the radiopharmaceutical. 3, 4

Recommended Next Steps

Immediate Laboratory Assessment

Measure TSH, free T4, and free T3 to determine the phase of thyroid dysfunction. 1 This will distinguish between:

• Thyrotoxic phase: Low/suppressed TSH with elevated free T4/T3 (occurs in early thyroiditis)
• Hypothyroid phase: Elevated TSH with low free T4 (occurs after thyrotoxic phase resolves)
• Euthyroid state: Normal values (may indicate recovery or subclinical disease)

Differentiate Thyroiditis from Other Causes

Check thyroid autoantibodies and consider clinical context:

• TSH receptor antibody (TRAb) or thyroid stimulating immunoglobulin (TSI) to exclude Graves disease, which would show increased uptake, not decreased 1
• Thyroid peroxidase (TPO) antibodies for Hashimoto thyroiditis 1
• Thyroglobulin level: Elevated in destructive thyroiditis due to follicular cell damage, but suppressed in factitious thyrotoxicosis 1

Obtain detailed medication history focusing on:

• Recent iodinated contrast exposure (within 4-8 weeks) 2
• Amiodarone use 2
• Immune checkpoint inhibitors (anti-PD1/PD-L1, anti-CTLA-4) 1
• Exogenous thyroid hormone intake 2

Clinical Assessment for Thyroiditis Type

Assess for neck pain and systemic symptoms:

• Subacute (de Quervain) thyroiditis: Painful, tender thyroid with fever, elevated ESR/CRP 1
• Painless/lymphocytic thyroiditis: Asymptomatic or mild symptoms, no tenderness 1
• Drug-induced: Temporal relationship to medication initiation 1

Role of Additional Imaging

Thyroid ultrasound with Doppler can be complementary to distinguish destructive from hyperfunctioning causes:

• Decreased vascularity on Doppler supports thyroiditis (destructive process) 2
• Increased vascularity suggests Graves disease or toxic adenoma (hyperfunctioning) 2

However, the radionuclide scan is preferred over Doppler because it directly measures thyroid activity rather than inferring it from blood flow. 2

Management Algorithm

If Thyrotoxic Phase Confirmed (Low TSH, High T4/T3)

Thyroiditis is self-limiting and requires only symptomatic treatment:

• Prescribe non-selective beta blockers (preferably with alpha-blocking capacity like carvedilol) for palpitations, tremors, anxiety 1
• Avoid antithyroid drugs (methimazole, propylthiouracil) as they are ineffective in destructive thyroiditis 1
• Monitor thyroid function every 2-3 weeks during thyrotoxic phase, which typically lasts approximately one month 1

Monitoring for Progression to Hypothyroidism

Hypothyroidism develops an average of 1 month after the thyrotoxic phase:

• Watch for symptoms: unexplained fatigue, weight gain, cold intolerance, constipation 1
• Start levothyroxine when TSH is elevated and free T4 is low 1
• Full replacement dose is approximately 1.6 mcg/kg/day for patients <70 years without cardiac disease 1
• For elderly or cardiac patients, start with 25-50 mcg/day and titrate gradually 1

Before initiating levothyroxine, rule out concurrent adrenal insufficiency, as thyroid hormone replacement can precipitate adrenal crisis. 1

Specialist Referral

Endocrinology consultation is recommended in all cases of suspected or confirmed thyroiditis to ensure appropriate management of evolving thyroid dysfunction. 1

Common Pitfalls to Avoid

• Do not perform radioiodine uptake scan if hypothyroidism is already established, as all causes show decreased uptake and imaging provides no additional diagnostic value 2
• Do not interpret reduced uptake as hypothyroidism without checking thyroid function tests, as patients may be thyrotoxic in the destructive phase of thyroiditis 1
• Do not start antithyroid medications for thyroiditis-induced thyrotoxicosis, as the gland is not overproducing hormone but rather releasing preformed hormone from damaged follicles 1
• Do not assume permanent hypothyroidism immediately, though most thyroiditis cases (especially drug-induced) lead to permanent hypothyroidism requiring lifelong replacement 1