What is the appropriate evaluation and treatment approach for patients with concerns about thyroid uptake?

Thyroid Uptake Evaluation and Treatment Approach

Thyroid uptake testing should be performed primarily for patients with suspected thyrotoxicosis to determine the underlying cause, with radionuclide uptake and scan being the preferred initial imaging modality for this purpose. 1

Initial Evaluation of Thyroid Function

First-Line Testing

• TSH measurement is the preferred initial test for suspected thyroid dysfunction 2
• If TSH is abnormal, follow with free T4 measurement
• If TSH is undetectable and free T4 is normal, obtain T3 level to evaluate for T3 toxicosis

When to Consider Thyroid Uptake Testing

Thyroid uptake testing is indicated in the following scenarios:

1. Thyrotoxicosis evaluation: To differentiate between causes such as:

   • Graves' disease
   • Toxic adenoma
   • Toxic multinodular goiter
   • Subacute thyroiditis 1

2. Planning radioactive iodine (RAI) treatment: To determine appropriate dosing for patients requiring RAI therapy 1

3. Goiter with thyrotoxicosis: To confirm that the entire goiter consists of thyroid tissue 1

Preferred Imaging Modalities

Radionuclide Uptake and Scan

• Iodine-123 (I-123) is preferred over iodine-131 (I-131) due to superior imaging quality 1
• Helps differentiate between causes of thyrotoxicosis:
  • Increased uptake: Graves' disease, toxic adenoma, toxic multinodular goiter
  • Decreased uptake: Thyroiditis, exogenous thyroid hormone ingestion

Ultrasound (US)

• Best imaging study to evaluate thyroid morphology
• Useful adjunct to radioiodine uptake scan
• Can confirm presence of nodules and evaluate for suspicious features of malignancy
• Provides thyroid dimensions for planning RAI treatment 1
• Should be performed if a palpable thyroid nodule or goiter is detected 3

Other Imaging Modalities

• Doppler US: May be an alternative to nuclear medicine for separating thyrotoxicosis due to overactive thyroid from destructive causes, but radionuclide uptake study is still preferred 1
• FDG-PET/CT: Not recommended as initial imaging for goiter evaluation but useful for detecting recurrence or metastases in differentiated thyroid carcinoma with high sensitivity (80-90%) 1

Treatment Approach Based on Uptake Results

Normal or Increased Uptake (Graves' disease, toxic nodules)

• Treatment options include:
  • Antithyroid medications
  • Radioactive iodine therapy
  • Surgery (thyroidectomy)

Decreased Uptake (Thyroiditis, exogenous hormone)

• Supportive care with beta-blockers for symptomatic relief
• No role for antithyroid medications or radioactive iodine

Persistent or Recurrent Disease After Treatment

• For differentiated thyroid cancer patients with evidence of persistent disease:
  • Imaging techniques for disease localization
  • Therapeutic doses of I-131 1
  • Locoregional disease: Combination of surgery and radioiodine therapy
  • External beam radiotherapy when complete surgical excision not possible or no significant radioiodine uptake 1

Follow-Up After Treatment

Short-term Follow-up

• 2-3 months after initial treatment: Thyroid function tests (FT3, FT4, TSH) to check adequacy of therapy 1
• 6-12 months: Comprehensive follow-up including:
  • Physical examination
  • Neck ultrasound
  • Serum thyroglobulin (Tg) measurement 1

Long-term Follow-up

• Annual physical examination
• Basal serum Tg measurement
• Neck ultrasound 1

Common Pitfalls and Caveats

1. Diagnostic accuracy: Radionuclide uptake directly measures thyroid activity, while Doppler US infers it based on blood flow, making radionuclide uptake more reliable 1

2. Timing considerations: Recent iodine exposure (CT contrast, amiodarone, supplements) can interfere with uptake studies and lead to false results

3. Hypothyroidism evaluation: There is no role for uptake imaging in the workup of hypothyroidism in adults, as all causes will have decreased radioiodine uptake 1

4. Recurrence monitoring: Even after successful treatment, local or distant recurrence may develop in late follow-up, even 20 years after initial treatment, necessitating long-term surveillance 1

5. Pregnancy considerations: Radioactive iodine is contraindicated during pregnancy and breastfeeding

By following this structured approach to thyroid uptake evaluation and treatment, clinicians can effectively diagnose and manage patients with thyroid disorders while minimizing unnecessary testing and treatment.