Normal T4 Level
A normal thyroxine (T4) level is within the laboratory reference range, typically around 4.5 to 10.0 mIU/L, though specific reference ranges may vary slightly between laboratories. 1
Understanding T4 and Thyroid Function
T4 (thyroxine) is one of the primary hormones produced by the thyroid gland. When evaluating thyroid function, it's important to understand:
- The thyroid gland produces both T4 (80% of production) and T3 (20% of production) 2
- Most T4 in circulation comes directly from the thyroid gland 2
- In serum, most T4 is bound to proteins, with only about 0.02% circulating as free T4 (FT4) 2
- Free T4 is the biologically active form and correlates best with hormonal activity 3
Clinical Interpretation of T4 Levels
When interpreting T4 levels, consider the following:
- Normal T4 levels exist within a laboratory reference range, but this range is based on statistical distribution across the general population rather than association with symptoms or outcomes 1
- Individual variations in T4 are actually quite narrow compared to the broader population reference ranges 4
- Each person has their own unique thyroid "set point" within the reference range 4
- The width of individual 95% confidence intervals for T4 is approximately half that of the group reference range 4
Relationship Between T4 and Other Thyroid Tests
T4 levels should be interpreted in context with other thyroid function tests:
- TSH (thyroid-stimulating hormone) is the primary screening test for thyroid dysfunction 1
- In subclinical hypothyroidism, TSH is elevated (>4.5 mIU/L) but T4 remains normal 1
- In overt hypothyroidism, TSH is elevated and T4 is low 1
- In subclinical hyperthyroidism, TSH is low (<0.4 mIU/L) but T4 and T3 remain normal 1
- In overt hyperthyroidism, TSH is low or undetectable and T4 or T3 is elevated 1
Important Clinical Considerations
- Multiple tests should be performed over a 3-6 month interval to confirm abnormal findings 1
- Follow-up testing of T4 levels in persons with persistently abnormal TSH levels can differentiate between subclinical and overt thyroid dysfunction 1
- Free T4 measurements have largely replaced total T4 measurements in clinical practice due to their higher diagnostic performance 2
- When thyroid status is unstable (e.g., first months of thyroid treatment, altered levothyroxine dose), TSH may be misleading and free hormone measurements are more reliable 2
Pitfalls in T4 Interpretation
- Laboratory reference ranges may be insensitive to changes that are significant for an individual due to the narrow individual variation compared to population ranges 4
- A T4 value within laboratory reference limits is not necessarily normal for a specific individual 4
- In patients on levothyroxine replacement therapy with normal TSH, free T4 is often higher than in untreated euthyroid individuals 5
- Relying solely on TSH levels for assessing levothyroxine replacement therapy may not be adequate 6
- Free hormone measurement remains technically demanding, especially in patients with severe non-thyroid illness 2
By understanding normal T4 levels in context with other thyroid function tests, clinicians can better assess thyroid function and make appropriate treatment decisions to optimize patient outcomes.