What does it measure?
The calculation divides free T3 by free T4 after accounting for their units. Because tissues convert some T4 into T3, researchers study the ratio as one possible signal of thyroid hormone handling. It does not directly measure that conversion in every tissue. [1]
Before reading the calculation
- Inputs
- Free T3 divided by free T4. Record both units and the laboratory method before comparing results.
- Context
- Retain TSH and both hormone values. A ratio removes information about their individual levels.
What can the result mean?
A lower ratio may reflect lower free T3, higher free T4, or both. Illness, medicines, and reduced energy intake can change the pattern. A higher ratio is not automatically healthier. Observational links with health outcomes do not show that changing the ratio improves health. [1]
Free T3 divided by free T4. Record both units and the laboratory method before comparing results.
- Context
- Retain TSH and both hormone values. A ratio removes information about their individual levels.
When is it useful, and how is it read?
This calculation is mainly a research description. When symptoms or an unusual thyroid result need explanation, return to TSH, free T4, the individual T3 result, and the clinical situation. Illness-related changes and assay differences can influence the ratio without identifying a new thyroid disorder. [1] [2]
What can affect it?
Different assays and reporting units can produce different values. Thyroid medicines, especially preparations containing T3, make timing particularly important. The ratio cannot replace TSH and the individual hormone results or establish a diagnosis of impaired conversion. [1]
A ratio does not directly measure hormone conversion
FT3 divided by FT4 summarizes two blood concentrations. It does not directly measure how fast your tissues convert T4 to T3, and an observational association does not establish a treatment target. [1]
Questions you might have
Does this ratio diagnose poor T4-to-T3 conversion?
No. An observational association with health outcomes does not establish a diagnostic conversion test for one person. [1]
Does raising the ratio improve health?
That has not been shown by the cited observational study. An association is different from evidence that changing a number helps. [1]
What comes next?
If thyroid symptoms are a concern, review established tests and the wider medical picture with your clinician. Do not adjust medication or add iodine or other supplements to pursue a ratio target. A clear clinical question is more useful than a calculated number without an agreed decision behind it.
Would this ratio change my care beyond what TSH, free T4, and the clinical assessment already show?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
