What laboratory tests are included in a complete thyroid panel?

Complete Thyroid Panel Laboratory Tests

A full thyroid panel should include TSH, free T4, free T3, anti-thyroid peroxidase antibodies (TPO-Ab), and anti-thyroglobulin antibodies (Tg-Ab) to comprehensively assess thyroid function and identify autoimmune thyroid disease. 1, 2

Core Components of a Complete Thyroid Panel

Primary Screening Tests

• Thyroid-Stimulating Hormone (TSH) is the single most sensitive screening test for thyroid dysfunction, with sensitivity above 98% and specificity greater than 92% 3, 4, 5
• Free Thyroxine (Free T4 or FT4) measures the metabolically active unbound hormone and is essential for distinguishing subclinical from overt thyroid dysfunction 3, 2, 6
• TSH and free T4 should be measured simultaneously as the cornerstone tests for evaluating thyroid function 2

Thyroid Hormone Assessment

• Free Triiodothyronine (Free T3 or FT3) may be useful in cases with discordant TSH and FT4 results or in highly symptomatic patients, though it is not routinely necessary for initial assessment 2, 7
• Free T3 is particularly important when hyperthyroidism is suspected but free T4 is normal, as T3 toxicosis can occur 3, 7
• T3 measurement does not add information to the interpretation of thyroid hormone levels in subjects with hypothyroidism on levothyroxine replacement therapy 8

Autoimmune Markers

• Anti-thyroid peroxidase antibodies (TPO-Ab) are the hallmark laboratory markers for Hashimoto's thyroiditis and should be tested in all suspected cases of autoimmune thyroid disease 2, 5
• Anti-thyroglobulin antibodies (Tg-Ab) confirm the autoimmune nature of thyroid disease and are critical for comprehensive evaluation 2, 5
• Anti-TPO antibodies are more predictive of future thyroid dysfunction than anti-thyroglobulin antibodies in patients with type 1 diabetes 1
• The presence of thyroid autoantibodies indicates autoimmune thyroid disease is already present, even if thyroid function remains normal 1

When to Measure Each Component

Initial Diagnostic Workup

• Measure TSH first as the primary screening test for suspected thyroid dysfunction 3, 4
• If TSH is abnormal, measure free T4 to distinguish between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4) 4
• Check anti-TPO antibodies to confirm autoimmune etiology, which predicts higher progression risk to overt hypothyroidism (4.3% per year vs 2.6% in antibody-negative individuals) 1, 4

Special Clinical Scenarios

• Free T3 measurement is particularly valuable when:

  • Hyperthyroidism is suspected but free T4 is normal (T3 toxicosis) 3, 7
  • TSH and free T4 results are discordant 2
  • Patients are highly symptomatic despite normal TSH and free T4 2

• Free T3 should NOT be routinely measured when:

  • Assessing adequacy of levothyroxine replacement in hypothyroid patients 8
  • TSH is elevated and free T4 is low (overt hypothyroidism is already confirmed) 9

Important Caveats and Pitfalls

Methodological Considerations

• Free hormone measurements using equilibrium dialysis/RIA are considered the reference method, though routine clinical laboratories use automated immunoassays 7
• Free thyroid hormone measurement remains technically demanding, especially in sera from severe non-thyroid ill patients with low serum thyroxine binding capacity 7
• Heterophilic antibodies can cause falsely high TSH concentrations in some assays 2

Clinical Context Matters

• Medication effects such as glucocorticoids and dopamine can affect TSH levels 2
• Thyroid function may appear abnormal in the absence of actual thyroid dysfunction during pregnancy and in critical illness 5
• When the thyroidal status is unstable (first months of thyroid treatment, altered levothyroxine dose, subacute thyroiditis) or when hypothalamic-pituitary function is disturbed (central hypothyroidism), TSH determination is diagnostically misleading and only free hormone measurements are reliable 7

Total Hormone Measurements

• Total T4 and total T3 measurements are influenced by variations in thyroid hormone-binding proteins, especially thyroxine-binding globulin (TBG), making them less reliable than free hormone measurements 6
• Total hormone measurements remain useful for research studies or in cases of severe hyperthyroidism 7

Monitoring Protocol After Diagnosis

• TSH and free T4 should be monitored every 4-6 weeks initially when starting treatment 2
• Monitor every 6-8 weeks while titrating hormone replacement 2
• Once stable, monitor every 6-12 months 2
• Women with Hashimoto's thyroiditis who are pregnant or planning pregnancy require more frequent monitoring due to increased thyroid hormone requirements during pregnancy 2