Laboratory Evaluation of a Thyroid Mass
Begin with serum TSH measurement as the single most important initial laboratory test for any thyroid mass, as this result determines the entire subsequent diagnostic pathway. 1, 2
Initial Laboratory Testing
TSH is the mandatory first test that must be obtained before proceeding with any imaging or additional workup. 1, 2, 3 The TSH level stratifies patients into three distinct pathways:
- Normal TSH (euthyroid): Proceed directly to ultrasound evaluation without additional thyroid function tests 1
- Low/suppressed TSH (<0.4 mIU/L): Indicates possible hyperfunctioning nodule requiring both ultrasound and radioiodine uptake scan 1, 2
- Elevated TSH: Suggests hypothyroidism; imaging generally not indicated unless mass effect symptoms present 1
Additional Testing Based on TSH Results
For Suppressed TSH (Thyrotoxicosis)
When TSH is low, measure free T4 (FT4) and free T3 (FT3) to confirm and quantify the degree of hyperthyroidism. 4, 5
- Free hormone measurements are superior to total hormone levels because they reflect the biologically active fraction 5
- FT4 elevation confirms overt hyperthyroidism 4, 5
- FT3 should be measured specifically when hyperthyroidism is suspected, as T3 toxicosis can occur with normal T4 4, 2
Critical pitfall: Do not skip thyroid antibody testing in this population, as coexisting autoimmune disease (Graves' disease) requires different management than toxic nodular disease. 4
For Normal or Elevated TSH
No additional thyroid function tests are routinely needed for nodule evaluation in euthyroid patients. 1, 2 The focus shifts entirely to structural assessment via ultrasound and potential fine-needle aspiration based on sonographic features. 2
- T3 measurement adds no diagnostic value in euthyroid patients with nodules 6, 3
- Free T4 measurement is unnecessary if TSH is normal, unless central hypothyroidism is suspected 3
Thyroid Autoantibody Testing
Measure anti-thyroid peroxidase (anti-TPO) antibodies when evaluating thyroid masses, particularly if:
- TSH is elevated or trending upward (identifies Hashimoto's thyroiditis) 4
- Patient has type 1 diabetes or other autoimmune conditions (16-fold increased risk) 4
- There is diffuse thyroid enlargement rather than discrete nodule 4
Anti-thyroglobulin antibodies may also be measured but are less sensitive than anti-TPO for autoimmune thyroid disease. 4
Serum Thyroglobulin
Do not measure serum thyroglobulin (Tg) in the initial evaluation of a thyroid mass. 4 This test is reserved exclusively for:
- Post-thyroidectomy surveillance in differentiated thyroid cancer patients 4
- Monitoring for recurrence after radioiodine ablation 4
Thyroglobulin has no role in determining malignancy risk of an intact thyroid nodule and will be elevated in most benign thyroid conditions. 4
Common Pitfalls to Avoid
Do not order a "complete thyroid panel" reflexively. 1, 3 This wastes resources and generates confusing results:
- T3 measurement in euthyroid patients on levothyroxine or with nodules provides no useful information 6, 3
- Total T4 and T3 are inferior to free hormone measurements and affected by binding protein abnormalities 5
- Reverse T3 has no role in thyroid nodule evaluation 7
Do not proceed to radioiodine scanning without first checking TSH. 1, 2 Uptake scans in euthyroid patients have extremely low diagnostic yield and expose patients to unnecessary radiation. 1
Do not use thyroid function tests to predict malignancy. 2 Thyroid cancer occurs with equal frequency in euthyroid, hypothyroid, and hyperthyroid patients—TSH levels do not stratify cancer risk. 2
Algorithmic Summary
- Measure TSH first in every patient with a thyroid mass 1, 2, 3
- If TSH suppressed: Add FT4, FT3, then proceed to ultrasound followed by radioiodine uptake scan 4, 1, 2
- If TSH normal or elevated: Proceed directly to ultrasound; consider anti-TPO antibodies if TSH elevated 4, 1, 2
- Never measure: Thyroglobulin, reverse T3, or perform uptake scanning in euthyroid patients 4, 1, 7