Free T4 measures the unbound, active fraction of the thyroids main product, the hormone every cell converts to T3. Read together with TSH it separates primary thyroid disease from the rare pituitary causes.
Standard range versus optimal range
| Framework | Free T4 value | Source |
|---|---|---|
| Standard lab reference | about 0.8-1.8 ng/dL | Common laboratory range |
| Overt hypothyroidism | low free T4 with high TSH | ATA guideline |
| Overt hyperthyroidism | high free T4 with suppressed TSH | ATA guideline |
When it runs high
High free T4 confirms an overactive thyroid or excessive replacement dose. Sustained elevation accelerates bone turnover and strains the heart, and the combination of high free T4 with suppressed TSH defines overt hyperthyroidism.
- Graves disease
- Toxic nodular goiter
- Thyroiditis with hormone release
- Excess levothyroxine
- Rare TSH-producing pituitary tumors, where TSH stays inappropriately normal
When it runs low
Low free T4 means thyroid hormone supply is inadequate for tissue needs. With high TSH it confirms primary hypothyroidism, while low free T4 with an inappropriately normal TSH points to pituitary or hypothalamic failure.
- Hashimoto thyroiditis or post-surgical hypothyroidism
- Central hypothyroidism from pituitary disease
- Severe non-thyroidal illness, where conversion shifts toward reverse T3
- Iodine deficiency
- Insufficient replacement dose in treated patients
The workhorse hormone, measured unbound
The thyroid releases mostly T4, a prohormone with a long half-life that tissues convert into the active T3 as needed. Over 99 percent of T4 rides on binding proteins, and only the free fraction can enter cells, which is why the free measurement replaced total T4 decades ago. Free T4 moves more slowly than TSH, so the pair answers different questions, TSH shows what the pituitary wants, free T4 shows what the gland delivered.
The pattern grid clinicians use
Four combinations cover nearly everything. High TSH with low free T4 is overt primary hypothyroidism. Suppressed TSH with high free T4 is overt hyperthyroidism. High TSH with normal free T4 is subclinical hypothyroidism, and suppressed TSH with normal free T4 is subclinical hyperthyroidism. The fifth pattern, low free T4 with a non-elevated TSH, is the red flag for central disease and the reason free T4 belongs in any ambiguous workup.
Interpreting with context
Pregnancy raises binding proteins and changes targets, which is why trimester-specific ranges exist. Biotin interferes with many assays, falsely raising free T4, and severe illness shunts T4 metabolism in ways that make single measurements misleading. When symptoms, TSH and free T4 tell different stories, repeating the panel on a different platform usually resolves the conflict.
Related calculators
Related biomarkers TSH · Free T3 · Reverse T3