Patient with FDG-avid lesions on PET scan in the thyroid, stomach, and iliac bone—what laboratory tests should be ordered?

Laboratory Workup for FDG-Avid Lesions in Thyroid, Stomach, and Iliac Bone

Order a comprehensive metabolic panel including serum calcium, parathyroid hormone (PTH), thyroglobulin, calcitonin, complete blood count with differential, and inflammatory markers (ESR/CRP) as your initial laboratory assessment.

Algorithmic Approach to Laboratory Testing

Thyroid Lesion Workup

• Thyroid-stimulating hormone (TSH) and free T4 1, 2
• Serum thyroglobulin - essential if differentiated thyroid cancer is suspected 3, 4
• Calcitonin - critical for medullary thyroid carcinoma evaluation 1, 2
  • Some guidelines recommend routine calcitonin screening for all thyroid nodules, though this remains controversial in the US 1
• Thyroglobulin antibodies - if thyroglobulin is measured, as antibodies interfere with interpretation 3, 4

Bone Lesion (Iliac) Evaluation

This is where calcium and PTH become critical - FDG-avid bone lesions can represent:

1. Primary hyperparathyroidism with brown tumors 5, 6

   • Serum calcium (>3.5 mmol/L or >140 mg/L suggests parathyroid carcinoma) 5
   • PTH levels (3-10 times upper limit of normal in parathyroid carcinoma) 5
   • Alkaline phosphatase (often elevated) 5
   • Critical pitfall: Brown tumors from severe hyperparathyroidism show FDG avidity and can be misinterpreted as metastases 5, 6

2. Metastatic disease (thyroid cancer to bone)

   • Elevated thyroglobulin with bone metastases 7
   • Bone-specific alkaline phosphatase 8

Gastric Lesion Workup

• Complete blood count - assess for anemia from gastric involvement 9
• Comprehensive metabolic panel - liver function, renal function 9
• HER2 testing (if gastric malignancy confirmed) 9
• MSI/MMR testing (if gastric malignancy confirmed) 9

Histiocytic Disorders Screening

Given the multifocal FDG-avid pattern, consider histiocytic neoplasms 10, 11, 12:

• Basic labs: CBC with differential, sodium, potassium, calcium, AST, ALT, alkaline phosphatase, bilirubin, BUN, creatinine, C-reactive protein, LDH 10
• Endocrine panel (for Erdheim-Chester disease or Langerhans cell histiocytosis):
  • Morning urine and serum osmolality (diabetes insipidus screening)
  • FSH, LH with testosterone (males) or estradiol (females)
  • Morning cortisol with ACTH
  • TSH and free T4
  • Prolactin
  • IGF-1 10

Priority Testing Sequence

First-tier labs (order immediately):

• Serum calcium and PTH
• TSH, free T4
• Thyroglobulin and thyroglobulin antibodies
• Calcitonin
• CBC with differential
• Comprehensive metabolic panel
• Alkaline phosphatase

Second-tier labs (based on initial results):

• If calcium/PTH elevated: Consider parathyroid carcinoma workup
• If thyroglobulin elevated: Evaluate for differentiated thyroid cancer with bone metastases
• If multisystem involvement suspected: Complete histiocytic disorder panel 10

Critical Clinical Pearls

The calcium-PTH axis is your diagnostic lynchpin here - severely elevated calcium (>3.5 mmol/L) with markedly elevated PTH (3-10x normal) strongly suggests parathyroid carcinoma, and the bone lesion may be a brown tumor rather than metastasis 5. This distinction fundamentally changes management.

FDG-avid thyroid lesions carry a 16-64% malignancy risk depending on the series 13, 14, 15, making thyroid-specific markers (thyroglobulin, calcitonin) essential even before tissue diagnosis.

The combination of thyroid, gastric, and bone involvement should also trigger consideration of systemic histiocytic disorders (Erdheim-Chester disease, Langerhans cell histiocytosis, Rosai-Dorfman disease), which require specific endocrine screening 10, 11, 12.