Most doctors only order TSH. Research shows Free T3, Free T4, and reverse T3 tell the rest of the story. Learn what a complete thyroid panel reveals.
Quick Summary TSH (Thyroid Stimulating Hormone) is the standard thyroid test ordered in most clinical settings. Research shows it is an indirect marker -- measuring the signal from the pituitary, not the actual thyroid hormones doing the work. A complete thyroid panel including Free T3, Free T4, and reverse T3 provides significantly more information. Educational article -- not medical advice.
The thyroid system operates through a feedback loop: hypothalamus releases TRH, pituitary releases TSH, thyroid produces T4, T4 converts to T3 in peripheral tissues, T3 drives cellular metabolism. TSH measures step 3. Free T3 measures the active hormone doing the actual work.
Someone can have a normal TSH with low Free T3 due to impaired T4-to-T3 conversion -- appearing normal on a standard test while experiencing thyroid deficiency symptoms.
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TSH -- Research reference: 0.5-2.5 mIU/L for optimized thyroid function. Standard lab ranges often run 0.4-4.0 mIU/L.
Free T4 -- Research reference: 1.0-1.5 ng/dL.
Free T3 -- The active thyroid hormone. Research reference: 3.0-4.0 pg/mL. Many practitioners consider Free T3 the most clinically relevant thyroid marker.
Reverse T3 (rT3) -- Under stress, caloric restriction, illness, or inflammation, T4 preferentially converts to inactive rT3. Research reference: rT3 below 15 ng/dL. The Free T3/rT3 ratio above 0.2 is commonly referenced in optimization literature.
TPO Antibodies -- Elevated TPO antibodies indicate Hashimoto thyroiditis -- the most common cause of hypothyroidism, frequently present years before TSH becomes abnormal. Research reference: below 34 IU/mL.
Thyroglobulin Antibodies (TgAb) -- A second autoimmune marker. Including both provides a more complete autoimmune picture.
GH peptides promote T4-to-T3 conversion. Research shows GH-deficient adults may see thyroid function improvement with GH restoration. GLP-1 clinical trials include thyroid monitoring as a safety measure. Caloric restriction elevates rT3 through adaptive thermogenesis -- tracking rT3 during active weight loss provides important context.
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My TSH is normal but I feel terrible -- what should I check?
Research supports checking Free T3, Free T4, rT3, and TPO antibodies when symptoms persist despite normal TSH.
What causes low Free T3 with normal TSH?
Research identifies caloric restriction, chronic stress, inflammation, selenium deficiency, and liver dysfunction as causes.
Is selenium important for thyroid function?
Yes. Research shows selenium is required for the enzyme that converts T4 to active T3.
Should I get thyroid antibodies tested even if my TSH is normal?
Research supports including TPO antibodies in a comprehensive thyroid panel. Hashimoto thyroiditis is often present for years with normal TSH.
How does stress affect thyroid function?
Research shows cortisol inhibits T4-to-T3 conversion and may suppress TSH signaling.
*This article is for educational purposes only. Consult a licensed healthcare provider for thyroid evaluation and treatment decisions.*
*Written by the MyProtocolStack team. Last updated: March 2026.*
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