What labs do I check for a thyroid nodule?

Laboratory Evaluation for Thyroid Nodules

Thyroid function tests, particularly TSH, should be performed as the first laboratory test for any patient with a thyroid nodule before proceeding to imaging studies. 1, 2

Initial Laboratory Evaluation

• TSH is the primary and most important initial laboratory test for evaluating thyroid nodules 1, 2, 3
• If TSH is subnormal (low), the patient has thyrotoxicosis, which will guide subsequent management 1, 2
• If TSH is normal or elevated, proceed with ultrasound evaluation of the nodule 1, 3, 4

Additional Laboratory Tests Based on TSH Results

For Low TSH (Subnormal)

• After ultrasound evaluation of thyroid morphology, a radionuclide thyroid uptake scan should be performed to determine if the nodule is hyperfunctioning 1, 2, 3
• Hyperfunctioning nodules ("hot" nodules) are rarely malignant and typically do not require biopsy 2, 5

For Normal or High TSH

• No additional routine laboratory tests are required before proceeding to ultrasound evaluation 1, 2
• Ultrasound will help characterize the nodule for risk of malignancy 1, 2

Pre-Procedure Laboratory Tests

If fine needle aspiration biopsy (FNAB) is indicated based on ultrasound findings, the following tests may be considered:

• Basic laboratory tests including complete blood count 1
• Coagulation function assessment (especially important if patient is on anticoagulant therapy) 1
• Blood biochemistry analysis 1
• Blood type determination 1

Special Considerations

• Calcitonin measurement may be considered in the evaluation of thyroid nodules to screen for medullary thyroid carcinoma, though routine measurement is not universally recommended 1
• For patients with multinodular goiter, thyroid function tests should be performed first, followed by ultrasound evaluation of suspicious nodules 1

Common Pitfalls to Avoid

• Proceeding directly to radionuclide uptake scan without first checking TSH levels 2, 6
• Using radionuclide scanning to determine malignancy in euthyroid patients with nodules 1, 2
• Failing to discontinue anticoagulant medications before FNAB if indicated 1
• Overreliance on laboratory tests alone without correlating with ultrasound findings 2, 7

Algorithm for Laboratory Evaluation of Thyroid Nodules

1. Measure serum TSH level 1, 2, 3
2. If TSH is low: Perform ultrasound followed by radionuclide uptake scan 1, 2
3. If TSH is normal or high: Proceed with ultrasound evaluation 1, 2, 3
4. Based on ultrasound findings, determine if FNAB is indicated 1, 7
5. If FNAB is indicated, perform necessary pre-procedure laboratory tests 1

Remember that laboratory tests are just one component of the evaluation of thyroid nodules, and imaging studies, particularly ultrasound, play a crucial role in characterizing nodules and determining the need for biopsy 1, 2, 7.