Interpretation of Your Thyroid Function Tests
Your thyroid function tests show a pattern consistent with low thyroid hormone binding capacity (likely low thyroxine-binding globulin), not true hypothyroidism, and no treatment is needed—simply repeat TSH and free T4 in 3-6 weeks to confirm stability.
Understanding Your Results
Your results show:
- TSH 2.17 µIU/mL – solidly within the normal reference range of 0.45-4.5 mIU/L 1
- Total T4 6.6 µg/dL – below the typical reference range
- T3 uptake 16% – low (normal is typically 25-35%)
- Free thyroxine index (FTI) 1.1 – normal
What This Pattern Means
The combination of low total T4 with low T3 uptake and normal FTI indicates decreased thyroid hormone binding protein capacity, not thyroid disease 2, 3, 4. The T3 uptake test measures how much binding protein is available—a low value means you have less binding protein than average 3, 4. When binding proteins are low, total T4 falls proportionally, but the free (active) hormone remains normal 2, 5.
The free thyroxine index corrects for this binding protein variation 3, 4. Your FTI of 1.1 is calculated from total T4 × T3 uptake ratio, and this value falls within the normal range, confirming adequate free thyroid hormone 3, 4.
Most importantly, your TSH of 2.17 mIU/L is completely normal 1. TSH has 98% sensitivity and 92% specificity for detecting thyroid dysfunction 6, 1. A normal TSH definitively excludes both overt and subclinical thyroid disease in the vast majority of cases 1.
Recommended Next Steps
Immediate Action
Repeat TSH and measure free T4 (not total T4) in 3-6 weeks 1. Free T4 measurement directly assesses the active hormone fraction and is not affected by binding protein variations 2, 5. This will confirm that your thyroid function is truly normal.
Why Confirmation Testing Is Important
- 30-60% of mildly abnormal thyroid tests normalize spontaneously on repeat testing 1
- TSH can vary by up to 40-50% day-to-day due to physiological factors 1
- Acute illness, medications, or recent iodine exposure can transiently affect results 6, 1
What to Expect
If repeat testing shows normal TSH (0.45-4.5 mIU/L) and normal free T4, no further evaluation or treatment is needed 1. Your low total T4 and T3 uptake simply reflect individual variation in binding proteins, which has no clinical significance when free hormone levels are normal 2, 3, 5.
Common Causes of Low Binding Proteins
While not requiring treatment, low thyroid hormone binding capacity can occur with:
- Severe systemic illness 5
- Malnutrition or low protein states 5
- Nephrotic syndrome 5
- Certain medications (androgens, glucocorticoids) 6
Critical Pitfalls to Avoid
Do not initiate levothyroxine based on low total T4 alone when TSH is normal 1. Total hormone measurements are unreliable for diagnosing thyroid disease because they are heavily influenced by binding protein concentrations 2, 3, 4. Treatment based on total T4 in the setting of normal TSH leads to unnecessary therapy and risk of iatrogenic hyperthyroidism 1.
Do not assume thyroid disease is present without measuring free T4 or confirming with repeat TSH 1, 2. The free thyroxine index is a reasonable screening estimate, but direct free T4 measurement by modern immunoassay provides more reliable assessment, especially when binding protein abnormalities are suspected 2, 5.
When to Seek Further Evaluation
Recheck sooner (within 2 weeks) if you develop symptoms suggesting thyroid dysfunction:
- Severe fatigue, weight changes, temperature intolerance 1
- New cardiac symptoms (palpitations, chest pain) 1
- Significant changes in mental status 1
Otherwise, routine confirmation in 3-6 weeks is appropriate for asymptomatic individuals with these borderline results 1.