What tests are included in a thyroid panel?

What Tests Are Included in a Thyroid Panel

A standard thyroid panel includes TSH (thyroid-stimulating hormone) as the primary test, with free T4 (free thyroxine) added when evaluating thyroid dysfunction, and occasionally free T3 (free triiodothyronine) or total T3 in specific clinical scenarios. 1

Core Components of Thyroid Function Testing

Primary Screening Test

• TSH alone is the most sensitive initial test for detecting thyroid dysfunction, with sensitivity above 98% and specificity greater than 92% 2
• TSH measurement should be the first-line test for patients with suspected thyroid disease or for monitoring thyroid function 1

Standard Thyroid Panel Configuration

The typical thyroid panel ordered in clinical practice includes:

• TSH + Free T4: This combination is ordered in approximately 27% of thyroid function test requests and represents the standard comprehensive panel 3
• Free T4 measures the metabolically active, unbound thyroxine and is essential for distinguishing between subclinical and overt thyroid dysfunction 1, 4
• This two-test combination allows differentiation between primary thyroid disorders and central (pituitary/hypothalamic) causes 1

Additional Tests in Specific Situations

Free T3 or Total T3 should be added when:

• Evaluating suspected hyperthyroidism, as T3 may be elevated before T4 in early thyrotoxicosis 1, 4
• Confirming subclinical hyperthyroidism with low TSH (0.1-0.45 mIU/L) to exclude nonthyroidal illness 1
• Assessing patients with TSH <0.1 mIU/L, where measurement should occur within 4 weeks of initial testing 1

Total T4 and T3 uptake (RT3U) represent older methodology:

• These tests were historically used together to calculate a free thyroxine index (FTI) 5
• Modern free hormone assays have largely replaced this approach in clinical practice 4

Clinical Context for Test Selection

When TSH Alone Is Sufficient

• Initial screening in asymptomatic patients at risk for thyroid disease 1
• Monitoring stable patients on levothyroxine replacement therapy 2
• TSH alone accounts for approximately 52% of thyroid function test orders 3

When to Add Free T4

• Any abnormal TSH result requires free T4 measurement for complete evaluation 1
• Suspected central hypothyroidism (pituitary or hypothalamic dysfunction) where TSH may be inappropriately normal or low 1
• Monitoring patients during dose titration of thyroid hormone replacement 2

When to Add Free T3

• TSH between 0.1-0.45 mIU/L to exclude central hypothyroidism or nonthyroidal illness 1
• TSH <0.1 mIU/L to confirm hyperthyroidism and assess severity 1
• Suspected T3 toxicosis where T4 may be normal 4

Important Caveats About Thyroid Testing

Overutilization Concerns

• Free thyroid hormones (FT4 and FT3) are commonly ordered when not clinically indicated, accounting for 36% of all thyroid function tests 3
• Inappropriate ordering of thyroid panels (TSH + FT4) when TSH alone would suffice represents significant healthcare waste 3
• Current guidelines recommend TSH alone as the best initial test, yet comprehensive panels are frequently ordered unnecessarily 3

Methodological Considerations

• Free hormone assays are superior to total hormone measurements because they are not affected by variations in thyroid-binding proteins 4
• Conditions like pregnancy, TBG excess/deficiency, and nonthyroidal illness can affect total hormone levels while free hormones remain accurate 4
• Assay interferences and concurrent medications can cause discordant results that require clinical correlation 6

Antibody Testing (Not Part of Standard Panel)

• TPO antibodies and TRAb are diagnostic tests for autoimmune thyroid disease, not routine screening tests 6
• Thyroglobulin and thyroglobulin antibodies are tumor markers for thyroid cancer surveillance, not functional assessments 6
• These specialized tests should be ordered based on specific clinical indications, not as part of routine thyroid panels 6