Clarification: T3 Does Not Convert to T4
There is no test to evaluate T3 converting to T4 because this conversion does not occur in human physiology. The question appears to contain a fundamental misunderstanding of thyroid hormone metabolism.
The Actual Direction of Thyroid Hormone Conversion
T4 (thyroxine) converts to T3 (triiodothyronine), not the reverse. 1
- T4 is the primary hormone secreted by the thyroid gland and serves as a prohormone 1
- Peripheral tissues convert T4 to the more metabolically active T3 through 5'-monodeiodination 1
- In healthy individuals, approximately 72% of circulating T3 comes from peripheral conversion of T4, while only 28% is directly secreted by the thyroid gland 1
- The conversion rate of T4 to T3 in normal subjects is approximately 0.25 per day 1
Tests to Assess T4 to T3 Conversion
If you intended to ask about evaluating T4 conversion to T3, the following tests are relevant:
Primary Assessment
- Measure serum TSH and free T4 first as the initial screening tests for thyroid function 2
- Add free T3 measurement when assessing peripheral conversion, particularly if T3 thyrotoxicosis is suspected (occurs in ~5% of hyperthyroid cases) 2, 3
Calculating Conversion Efficiency
- The T3/T4 ratio can provide insight into conversion efficiency 4
- A low T3 relative to T4 suggests impaired peripheral conversion 1
- In sick euthyroid patients, the conversion rate drops significantly (from 0.25 to 0.13), with only 52.5% of circulating T3 deriving from peripheral conversion versus 72% in healthy individuals 1
Important Caveat for Patients on Levothyroxine
- T3 measurement adds no value in assessing levothyroxine (T4) over-replacement 5
- In patients taking levothyroxine, T3 levels remain normal even when TSH is suppressed and free T4 is elevated, making T3 an unreliable marker of over-treatment 5
- TSH and free T4 are sufficient for monitoring thyroid replacement therapy 6, 5
Clinical Context
The reverse T3 (rT3) to T3 ratio can indicate the balance between 5'-monodeiodination (producing active T3) and 5-monodeiodination (producing inactive rT3), with thyroid hormones themselves regulating this conversion 4. However, this is rarely measured in routine clinical practice.