What is the appropriate evaluation and management for a patient with diffuse involvement of the thyroid parenchyma?

Evaluation and Management of Diffuse Thyroid Parenchymal Involvement

Imaging is generally not indicated for the initial workup of diffuse thyroid parenchymal disease, as the diagnosis and management are primarily based on clinical presentation and laboratory testing (TSH, thyroid antibodies), not imaging findings. 1

Initial Diagnostic Approach

Laboratory Testing First

• Measure TSH as the initial test before ordering any imaging studies 2, 3
• If TSH is abnormal, proceed with:
  • Free T4 or free thyroxine index 3
  • Thyroid peroxidase antibodies (TPOAb) if hypothyroidism is present 3, 4
  • TSH receptor antibodies (TRAbs) if hyperthyroidism is present 2

When Imaging May Be Appropriate

Ultrasound with Doppler is the only imaging modality that may add diagnostic value, and only in specific scenarios 1, 2:

• Thyrotoxicosis with unclear etiology: Doppler ultrasound differentiates Graves' disease (hypervascular, diffuse enlargement) from destructive thyroiditis (hypovascular or decreased flow) 2, 5, 3
• Palpable nodules within diffuse disease: Ultrasound identifies nodules requiring risk stratification using ACR TI-RADS 2
• Suspected goiter with obstructive symptoms: Ultrasound confirms thyroid origin and characterizes size/morphology 1

Imaging Modalities: Appropriateness by Clinical Scenario

Ultrasound Thyroid

• Usually appropriate for suspected goiter or when nodules are palpable within diffuse disease 1
• May be appropriate for thyrotoxicosis when combined with Doppler to assess vascularity 2, 5
• Usually not appropriate for primary hypothyroidism workup, as imaging does not differentiate causes 1

Radioiodine Uptake and Scan

• May be appropriate only when distinguishing Graves' disease from destructive thyroiditis is necessary and clinical/laboratory findings are equivocal 3
• Shows high uptake in Graves' disease and toxic multinodular goiter 2, 5
• Shows low or absent uptake in all forms of thyroiditis (Hashimoto, subacute, postpartum) 5, 3
• I-123 is preferred over I-131 for superior image quality 1, 5
• Usually not appropriate for hypothyroidism, as all causes show decreased uptake 1, 3

CT and MRI Neck

• Usually not appropriate for diffuse thyroid disease workup 1
• May be appropriate only if substernal extension is suspected or there is concern for invasive malignancy 1, 2
• CT is superior to ultrasound for evaluating substernal goiter and tracheal compression 1, 2

FDG-PET/CT

• Usually not appropriate for any form of diffuse thyroid disease 1

Management by Specific Diagnosis

Hashimoto Thyroiditis (Chronic Lymphocytic Thyroiditis)

• Diagnosis: Elevated TPOAb with or without elevated TSH 3, 4
• Imaging findings (if performed): Diffusely hypoechoic, heterogeneous parenchyma on ultrasound 6, 7
• Treatment: Levothyroxine 1.6 mcg/kg/day for overt hypothyroidism in patients <70 years without cardiac disease; start 25-50 mcg/day in elderly or cardiac patients 3
• Surveillance: Patients with positive TPOAb have 4.3% annual risk of progression to overt hypothyroidism; recheck TSH and free T4 at 3,6, and 12 months, then annually 3

Graves' Disease

• Diagnosis: Low TSH, elevated free T4, positive TRAbs 2
• Imaging findings (if performed): Diffuse hypervascular goiter on Doppler ultrasound; high radioiodine uptake 2, 5
• Treatment options: Antithyroid drugs, radioiodine ablation, or total thyroidectomy 2
• Surgery indication: Total thyroidectomy provides immediate permanent cure with no recurrences and is now preferred over subtotal thyroidectomy 2

Subacute (De Quervain) Thyroiditis

• Diagnosis: Anterior neck pain, triphasic pattern (thyrotoxicosis → hypothyroidism → recovery) 5, 4
• Imaging findings (if performed): Hypovascular on Doppler; low radioiodine uptake 5, 3
• Treatment: Beta blockers (propranolol or atenolol) for hyperthyroid symptoms; NSAIDs or corticosteroids for thyroid pain 5, 3, 4
• Levothyroxine: Generally not needed during hypothyroid phase unless TSH >10 mIU/L or severe symptoms develop 3
• Monitoring: Recheck TSH and free T4 every 2-3 weeks to monitor transition between phases 3

Multinodular Goiter

• Diagnosis: Nodular thyroid enlargement, may be euthyroid or hyperthyroid 2
• Imaging: Ultrasound first-line; CT if substernal extension or obstructive symptoms present 1, 2
• Treatment: Observation for asymptomatic non-toxic goiter; surgery for obstructive symptoms, cosmetic concerns, or substernal extension 2

Critical Pitfalls to Avoid

• Do not order radioiodine uptake scan for Hashimoto thyroiditis or hypothyroidism workup, as it adds no diagnostic value 3
• Rule out adrenal insufficiency before starting levothyroxine in patients with suspected central hypothyroidism, as thyroid hormone can precipitate adrenal crisis 3
• Do not start levothyroxine during the hyperthyroid phase of subacute or postpartum thyroiditis, as this worsens thyrotoxicosis 3
• Do not assume hypothyroidism is permanent without reassessing thyroid function 3-6 months after the acute phase of thyroiditis 3
• Defer all radioiodine treatment for at least 6 weeks after administration of iodinated contrast medium 1

Surgical Considerations

When surgery is indicated for diffuse thyroid disease:

• Refer to high-volume thyroid surgeons (>100 thyroidectomies annually) with 4.3% complication rates versus 4-fold higher rates for low-volume surgeons 2
• Obtain baseline flexible laryngoscopy to document vocal-fold mobility before surgery 2
• Permanent hypocalcemia occurs in 1.1-3% of patients after total thyroidectomy 2