Thyroid
Total T4
LOINC 3024-7 · mcg/dL
Ranges
Reference range: 4.5–12 mcg/dL
What it is
Total T4 measures all the thyroxine in your blood, including the part bound to proteins and the small free, active fraction.
Why it matters
It is a long-standing measure of thyroid hormone output, often read with TSH.
What high values can mean
A higher value can indicate an overactive thyroid; it is also influenced by hormone-binding proteins (as in pregnancy).
What low values can mean
A lower value can indicate an underactive thyroid.
How to improve it
Thyroid hormone is managed with medication and monitoring; consistency keeps levels steady.
Food & lifestyle
Adequate iodine and selenium support the thyroid; dosing is guided by your clinician.
Preparing for the test
A blood test; fasting is not required — follow your lab's instructions.
When to talk to a doctor
Discuss abnormal Total T4 with a doctor, interpreted with TSH and Free T4.
Related markers
Sources
- MedlinePlus (NIH)
- Testing.com (AACC)
Content reviewed on June 26, 2026
Medical disclaimer
This content is for educational purposes only. It is not medical advice and does not diagnose, treat, or prevent any condition. Reference ranges vary between laboratories — always use the range printed on your report and discuss your results with a healthcare professional.
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