TSH and free T4 describe different parts of thyroid regulation. TSH is a signal from the pituitary gland. T4 is a hormone made by the thyroid. The word “free” identifies the portion of T4 that is not attached to binding proteins.
Read the labels before the arrows
| Label | What the name refers to |
|---|---|
| TSH | Thyroid-stimulating hormone, produced by the pituitary |
| Total T4 | Bound and unbound thyroxine together |
| Free T4 | Thyroxine not bound to transport proteins |
Total T4 and free T4 are not merely two spellings of the same assay. Binding proteins can change in circumstances such as pregnancy or with certain medicines, which is one reason the exact measurement matters.
Why they may be considered together
A feedback system links the pituitary and thyroid. Measuring the signal and a thyroid hormone can provide more context than looking at either label alone. Clinicians may also use other tests when they need to investigate the cause of an abnormal finding.
This relationship should not be reduced to a universal two-arrow diagnosis chart. Illness, treatment, pregnancy, laboratory methods, and less common clinical situations can complicate interpretation. The table identifies measurements; it does not tell you how to change a prescription.
Three checks when reading a report
First, confirm whether the report says free or total T4. Second, keep each value with its own units and laboratory interval. Third, note whether the test was performed during treatment or for an initial investigation.
Thyroid blood tests usually use a venous sample. Discuss medicines and supplements with the ordering clinician, and follow instructions for the full order. If a result seems inconsistent with symptoms or an earlier report, ask for an explanation rather than choosing a reference range from another website.
Sources
- NIDDK: Thyroid tests
Roles of TSH, free and total thyroid hormones, and antibody tests.
- MedlinePlus: TSH
Pituitary signaling, thyroid follow-up tests, and medicine discussion.