Common MML Test-Ordering Errors
Part 4: Overordered Tests
JAK2 on Blood and Bone Marrow
Free Thyroxine index (FTI) and T3 Uptake
Thyroxine (T4)
July 2010
Thyroxine, called T4, is the main thyroid hormone. T4 circulates in 2 forms, protein bound (which accounts for over 99% of T4) and free. Free T4 accounts for <1% of circulating T4, but is the biologically active form of thyroxine. Both bound and free forms of thyroxine are measured by total T4 assays. While in most situations the total T4 test is a relatively reliable indicator of thyroxine status, when thyroid hormone-binding proteins are abnormal, total T4 results may be misleading. For example, increases in thyroxine-binding proteins can cause increased Total T4 levels despite normal free T4 levels and despite clinically normal thyroid function.
Thyroxine (T4) |
Jump to section:
- Introduction
- Testing at Mayo Medical Laboratories
- Common MML Test-Ordering Errors
- Overordered Tests
- JAK2 Gene
- Outdated Thyroid Function Tests
- Thyroxine (T4)
- Assessing Thyroid Hormone Status in the Presence of Abnormal Binding Proteins
- Free Thyroxine Index (FTI)
- Free T4 Assays
- Free T4 Assay Summary
- Conclusion
- Questions?
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