What is the upper limit of normal for Free Triiodothyronine (FT3) in a 20-year-old?

Normal Reference Range for FT3 in a 20-Year-Old

The upper limit of normal for Free Triiodothyronine (FT3) in a 20-year-old is approximately 6.5 pmol/L (using the Amerlex-MAB kit) or 9.2 pmol/L (using the Enzymun-test FT3 kit), with significant variation depending on the specific assay method used. 1

Understanding FT3 Reference Ranges

• Reference ranges for thyroid hormones, including FT3, vary significantly based on the laboratory and assay method used 1
• FT3 levels gradually decrease with age, with higher values typically observed in adolescents and young adults compared to older adults 2
• In young adults, FT3 levels are significantly higher than in individuals over 19 years of age 2
• Males tend to have higher FT3 levels than females between the ages of 11 and 19 years 2

Assay-Specific Variations

• Different laboratory methods produce substantially different reference ranges:
  • Amerlex-MAB kit: 3.2-6.5 pmol/L 1
  • Enzymun-test FT3 kit: 5.4-9.2 pmol/L 1
• Inter-method differences of approximately 1 mIU/L can exist at high-normal concentrations 3
• The lack of standardization between FT3 assays is a significant clinical challenge 1

Clinical Considerations When Interpreting FT3 Results

• FT3 should be interpreted in conjunction with TSH and FT4 levels for comprehensive thyroid function assessment 4
• Normal FT3 with abnormal TSH or FT4 may indicate various thyroid disorders:
  • Normal FT3 with low TSH may suggest subclinical hyperthyroidism 4
  • Normal FT3 with high TSH and normal FT4 may indicate subclinical hypothyroidism 4
• FT3 levels can remain normal even when total T3 is low in non-thyroidal illness syndrome 5

Factors That May Affect FT3 Levels

• Medications such as amiodarone can alter thyroid hormone levels 6
• Hospitalization status may affect FT3 levels, with hospitalized patients potentially having different values than outpatients 1
• Certain medical conditions like adrenocortical cancer treated with mitotane can cause central hypothyroidism with low FT4 but normal FT3 7

Important Caveats

• Always check which assay method your laboratory uses, as reference ranges are method-specific 1, 3
• Consider repeating abnormal thyroid function tests over a 3-6 month interval to confirm persistent abnormalities 6
• When evaluating thyroid function in young adults, age-specific reference ranges should be used rather than adult ranges 2
• The FT3/FT4 ratio may provide additional diagnostic information in certain clinical scenarios 7