Patient-Centered Education for Free T4 and Total T3 Testing
What These Tests Measure
Free T4 (thyroxine) and total T3 (triiodothyronine) are thyroid hormone tests that help determine whether your thyroid gland is functioning properly. 1
- Free T4 represents the active, unbound portion of thyroxine in your blood (only 0.02% of total T4), which is the most reliable indicator of thyroid hormone production 1
- Total T3 measures both bound and free triiodothyronine, with about 80% produced from T4 conversion outside the thyroid gland 1
- Free T4 is more reliable than T3 for assessing thyroid function because T3 levels can be influenced by many non-thyroid conditions 1
Why Your Doctor Ordered These Tests
Your physician may be checking for:
- Hyperthyroidism (overactive thyroid): Elevated free T4 and/or total T3 with low TSH 2
- Hypothyroidism (underactive thyroid): Low free T4 with high TSH in primary thyroid failure 3
- T3 thyrotoxicosis: A condition where only T3 is elevated while T4 remains normal, occurring in about 5% of hyperthyroid patients 4, 2
- Central hypothyroidism: Low free T4 with low or normal TSH, indicating pituitary or brain dysfunction 3
What to Expect During Testing
- Blood will be drawn from your arm, typically in the morning around 8 AM for most accurate results 5
- No special preparation is usually required unless your doctor specifies otherwise 5
- Results are typically available within a few days 1
Understanding Your Results
If Free T4 is Low:
- With high TSH: Your thyroid gland itself is not working properly (primary hypothyroidism) 3
- With low/normal TSH: Your pituitary gland or hypothalamus may not be signaling your thyroid correctly (central hypothyroidism), requiring additional testing including morning cortisol levels and pituitary MRI 3
If Total T3 is Elevated with Normal Free T4:
- This may represent early hyperthyroidism, subacute thyroiditis, Graves' disease, or toxic nodular goiter 6
- Your doctor will likely recheck these tests every 2-3 weeks to monitor for changes 6
- Beta-blockers may be prescribed for symptom relief while monitoring 6
If Both Are Elevated:
- This indicates overt hyperthyroidism requiring treatment 2
- Further testing with thyroid antibodies and imaging may be needed 6
Important Monitoring Information
If you're already taking levothyroxine (thyroid hormone replacement):
- Free T4 is the most important test to monitor your treatment, not T3 7
- T3 levels do not reliably indicate whether you're taking too much thyroid medication 7
- Normal T3 levels can occur even when you're over-replaced with levothyroxine 7
If you have suspected central hypothyroidism:
- Never start thyroid hormone replacement before ruling out adrenal insufficiency, as this could trigger a life-threatening adrenal crisis 3
- Your doctor must check cortisol levels first 3
Follow-Up Testing Schedule
- During initial diagnosis or dose adjustments: Recheck every 2-3 weeks for unstable conditions or every 6-8 weeks for stable dose changes 6, 3, 1
- Once stable: Monitor every 6-12 months 3
- For suspected T3 thyrotoxicosis: Close monitoring every 2-3 weeks to detect transition to hypothyroidism 6
When to Contact Your Doctor
Seek immediate medical attention if you experience: