You get your thyroid blood report and see multiple rows: TSH, Free T3, Free T4 — perhaps Total T3 and Total T4 as well. What do these numbers mean, which one matters most, and when should you be concerned? Here is a straightforward explanation.
The Thyroid Hormone System
The thyroid gland produces hormones (T3 and T4) under instruction from the pituitary gland (via TSH). Think of it as a chain of command:
- Pituitary gland → produces TSH (the "order")
- Thyroid gland → receives TSH and produces T4 (the "product")
- Body tissues → convert T4 to T3 (the "active form")
TSH — The Master Control
TSH (Thyroid Stimulating Hormone) is the single most important and sensitive marker of thyroid function. Normal range: approximately 0.4–4.0 mIU/L (varies slightly by laboratory).
- TSH HIGH (e.g., 8.5) = pituitary working overtime because thyroid is underperforming = Hypothyroidism
- TSH LOW (e.g., 0.1) = pituitary suppressed because thyroid is overproducing = Hyperthyroidism
- TSH NORMAL = thyroid function is likely fine
Free T4 — The Main Thyroid Output
Free T4 (FT4) measures the unbound, active form of thyroxine circulating in the blood. Normal range: approximately 0.8–1.8 ng/dL. I test FT4 when TSH is abnormal to understand the severity and guide treatment decisions.
Free T3 — The Active Hormone
Free T3 (FT3) is the most biologically active thyroid hormone. Most T3 is produced from T4 conversion in tissues. I check FT3 when I suspect hyperthyroidism, when symptoms persist despite normal TSH and T4, or when monitoring specific treatment situations.
🩺 Which Tests Do You Actually Need?
For routine screening and monitoring on treatment: TSH alone is sufficient in most cases. For initial diagnosis or when TSH is abnormal: TSH + FT4. For suspected hyperthyroidism or treatment monitoring for hyperthyroidism: TSH + FT3 + FT4. Don't pay for the full thyroid panel (T3, T4, TSH, anti-TPO, thyroglobulin) every time — it's usually not necessary. Let your physician guide which tests are needed.
Reference Ranges: What's Normal?
- TSH: 0.4–4.0 mIU/L (pregnancy: 0.1–2.5 in first trimester)
- Free T4: 0.8–1.8 ng/dL
- Free T3: 2.3–4.1 pg/mL
Note: reference ranges vary slightly between laboratories. Always compare your result to the range printed on your specific report.
Confused By Your Thyroid Report?
Bring your report to the next visit. Dr. Chaurasia will explain every number and what it means for your treatment.