The HPT Axis — Why TSH Alone Isn't Enough
The hypothalamic-pituitary-thyroid (HPT) axis is a negative feedback loop: TRH from the hypothalamus stimulates pituitary TSH release, which drives thyroidal T4 and T3 production. TSH alone is the most sensitive thyroid status indicator — but it doesn't distinguish central hypothyroidism from primary hypothyroidism without concurrent free T4 measurement. Thyroglobulin is a thyroid tissue-specific protein useful for recurrence monitoring. Anti-TPO and anti-Tg autoantibodies identify the autoimmune etiology underlying most thyroid dysfunction. Multiplex measurement captures TSH, free hormones, thyroglobulin, and autoantibodies in one panel — providing the complete thyroid profile that TSH-only testing cannot deliver.
Thyroid Function Markers
- TSH: pituitary glycoprotein — the most sensitive thyroid status indicator. Elevated in primary hypothyroidism; suppressed in hyperthyroidism.
- Free T4 & Free T3: Free T4 is the primary thyroid product, converted peripherally to the more potent Free T3. Free hormone measurements are independent of binding protein (TBG) variations.
- Thyroglobulin (Tg): thyroid follicular cell-specific protein. Undetectable after total thyroidectomy — elevation indicates residual tissue or recurrence.
- Thyroid Autoantibodies: Anti-TPO — present in >90% of Hashimoto's and 70% of Graves'. Anti-Tg — present in 60–70% of Hashimoto's.