Normal Reference Ranges
Adult
0.8 - 1.8 ng/dL
Pediatric
0.9 - 2.0 ng/dL
Clinical Significance
The active, unbound fraction of circulating thyroxine. More accurate than total T4 because it is not affected by changes in thyroid-binding globulin (TBG) levels.
Causes of High
- Hyperthyroidism (Graves' disease)
- Toxic multinodular goiter
- Thyroiditis (early phase)
- Excessive levothyroxine therapy
Causes of Low
- Hypothyroidism (Hashimoto's)
- Iodine deficiency
- Pituitary failure