Which laboratory tests should be ordered to evaluate a thyroid nodule (and a possible pulmonary nodule)?

Laboratory Tests for Thyroid Nodule Evaluation

Serum TSH is the single best initial laboratory test and should be ordered first for all thyroid nodules 1, 2. This algorithmic approach then determines all subsequent testing based on the TSH result.

Initial Testing Algorithm

Step 1: Measure Serum TSH

• TSH is the mandatory first test for thyroid function assessment in all patients with thyroid nodules 1
• This single test determines the entire subsequent laboratory workup

Step 2: Follow-up Testing Based on TSH Result

If TSH is LOW (suppressed):

• Measure free T4 (FT4) and free T3 (FT3) 1
• These patients may have functional autonomy and will need thyroid scintigraphy (not a lab test, but critical next step)

If TSH is HIGH (elevated):

• Measure thyroid peroxidase antibodies (TPOAb) and FT4 1
• This evaluates for autoimmune thyroiditis

If TSH is NORMAL:

• No additional routine thyroid function tests are needed 1

Calcitonin Testing - Selective, Not Routine

Calcitonin should NOT be routinely measured in all thyroid nodules 2. However, it is specifically indicated in these situations:

• Personal or family history of medullary thyroid carcinoma (MTC) or MEN2 syndrome
• Thyroid nodule with diarrhea, flushing, or lymph node metastasis
• Suspicious nodule features on ultrasound or FNA
• Before any thyroid surgery or thermal ablation 2

When calcitonin is measured and moderately elevated (10-100 pg/mL), recheck in 3-12 months. Surgery should be considered if basal calcitonin exceeds 30 pg/mL in women or 60 pg/mL in men, or if there is >20% progression 2.

Tests That Should NOT Be Routinely Ordered

Thyroglobulin (Tg):

• Do not routinely measure serum thyroglobulin for initial thyroid nodule assessment 2
• Tg does not discriminate between benign and malignant nodules
• Only useful in specific scenarios: suspected metastatic differentiated thyroid cancer with distant metastases and no obvious primary tumor 2

Serum calcium and PTH:

• Not indicated for routine thyroid nodule evaluation
• Only relevant if concurrent hyperparathyroidism is suspected clinically

Regarding Pulmonary Nodules

The evidence provided does not support any specific laboratory testing for isolated pulmonary nodule evaluation. Pulmonary nodules require imaging-based assessment (CT chest), not laboratory tests. If both thyroid and pulmonary nodules are present and thyroid cancer with lung metastases is suspected, then thyroglobulin and anti-thyroglobulin antibodies become relevant 2, but only after establishing the thyroid primary diagnosis.

Common Pitfalls to Avoid

• Do not order a "thyroid panel" reflexively - start with TSH alone and let the result guide subsequent testing
• Do not measure calcitonin on everyone - this leads to false positives and unnecessary workups 2
• Do not measure thyroglobulin initially - it provides no diagnostic value for nodule characterization 2
• Do not skip TSH measurement - it is the foundation of the entire diagnostic algorithm and must be done first

Summary of Laboratory Orders

For ALL thyroid nodules:

1. Serum TSH (mandatory first test)

Then, based on TSH result:

• Low TSH → Add FT4 and FT3
• High TSH → Add TPOAb and FT4
• Normal TSH → No additional routine labs

Calcitonin only if:

• High-risk history (MTC, MEN2, family history)
• Suspicious clinical features (diarrhea, flushing, lymphadenopathy)
• Before any thyroid surgery or ablation