What Is a T4 Blood Test?
T4 is the primary hormone produced by your thyroid gland. It is a prohormone that converts to active T3.
Normal Ranges
- Total T4: 4.5–12.0 µg/dL
- Free T4: 0.8–1.8 ng/dL (preferred)
How It Is Used
- With TSH to diagnose thyroid conditions
- High TSH + low free T4 = hypothyroidism
- Low TSH + high free T4 = hyperthyroidism
Source: NLM MedlinePlus, American Association for Clinical Chemistry.
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