Normal Ranges for T3 and T4 Levels in Adults
In adults with no known thyroid disease, normal T4 (thyroxine) levels typically range from 12-22 pmol/L for free T4, and normal T3 (triiodothyronine) levels range from 1.3-2.6 nmol/L for total T3, though these values may vary slightly between laboratories. 1
Understanding Normal Thyroid Hormone Levels
Free T4 (Thyroxine) Reference Range
- Free T4 typically ranges from 12-22 pmol/L in most clinical laboratories, representing the biologically active, unbound fraction of thyroxine 1
- Total T4 measurements are less commonly used in modern practice, as free T4 has superior diagnostic performance for assessing thyroid function 2
- Free T4 originates entirely from thyroid gland secretion and represents approximately 80% of the thyroid's hormonal production 2
T3 (Triiodothyronine) Reference Range
- Total T3 typically ranges from 1.3-2.6 nmol/L in adults without thyroid disease 3
- Free T3 (FT3) measurements are also available, though less commonly ordered in routine practice 2
- Approximately 80% of circulating T3 is produced extrathyroidally from T4 deiodination, with only 20% directly secreted by the thyroid gland 2
Critical Context for Interpreting These Values
Why T4 is More Reliable Than T3
- T4 is a more reliable reflection of thyroid hormone production than T3 because T4 conversion to T3 can be influenced by various non-thyroidal conditions 2
- In patients on levothyroxine replacement therapy, T3 levels may remain normal even when patients are over-replaced, making T3 measurement of limited clinical value in this context 3
- The relationship between TSH and T4 is stronger (r = 0.73) than between TSH and T3 (r = 0.41), indicating T4 has more diagnostic value in thyroid dysfunction 4
Protein Binding Considerations
- Only 0.02% of T4 and 0.3% of T3 circulates in the free, biologically active form 2
- The remainder is bound to binding proteins, which is why free hormone measurements have superseded total hormone determinations in clinical practice 2
- Free hormone measurements remain technically demanding, especially in sera from severely ill patients with low serum thyroxine binding capacity 2
When These Values Are Clinically Significant
Subclinical Thyroid Dysfunction Definition
- Subclinical hypothyroidism is defined as elevated TSH with normal T4 and T3 levels, while subclinical hyperthyroidism is defined as low TSH with normal T4 and T3 levels 1
- This demonstrates that normal T4 and T3 levels do not exclude thyroid dysfunction when TSH is abnormal 1
Individual Variation vs. Population Ranges
- While population reference ranges are wide due to large differences between individuals, individual variation within a single person is narrow, typically less than 10% for T3 and T4 5
- This means that a person's "normal" may be at the high or low end of the reference range consistently over time 5
Important Clinical Caveats
TSH is the Primary Screening Test
- TSH measurement is the first-line test for thyroid function assessment, with sensitivity of 98% and specificity of 92% when used to confirm suspected thyroid disease 1
- Free T4 should be measured if TSH is abnormal to distinguish between subclinical and overt thyroid dysfunction 1
- T3 measurement is typically reserved for situations where hyperthyroidism is suspected but free T4 is normal 2
When Free Hormone Measurements Are Essential
- Free hormone measurements are the only reliable indicators of thyroid status when thyroidal status is unstable (first months of treatment, altered levothyroxine dose, subacute thyroiditis) or when hypothalamic-pituitary function is disturbed 2
- In these situations, TSH determination is diagnostically misleading 2
Laboratory Method Variability
- Reference ranges may vary between laboratories depending on the assay method used 2
- Equilibrium dialysis/RIA is considered the reference method for free hormone measurements, though routine clinical laboratories typically use automated immunoassays 2
- Always interpret results using the specific reference range provided by the testing laboratory 2