A simple ratio of free T3 to free T4 that offers context on thyroid hormone activation.
The fT3/fT4 ratio is a calculated value, not a separate blood test. It is free T3 divided by free T4, the two active thyroid hormones. It reflects how well the body converts the storage hormone T4 into the more active hormone T3.
The thyroid makes mostly T4, which tissues convert to the stronger T3. A lower fT3/fT4 ratio can suggest reduced conversion, sometimes seen in illness, stress, or low-calorie states. A higher ratio can appear in some thyroid conditions. The ratio adds nuance to standard thyroid testing and is read with TSH.
Aniva reads your result against research-backed ranges, not just the lab's wide normal. The reference shown below is specific to this biomarker.
There is no firmly established universal cut-off for this ratio, partly because it depends on the assay and units used. It is interpreted as a trend alongside the individual thyroid values.
| Pattern | Interpretation |
|---|---|
| Lower ratio | Reduced T4 to T3 conversion; seen in illness, stress, low-calorie states |
| Higher ratio | Relatively more active T3; seen in some thyroid conditions |
Thresholds vary by assay. Source: American Thyroid Association, thyroid function tests.
Acute illness lowers T3 (a normal protective response), which drops the ratio without thyroid disease. Some medicines (including amiodarone, steroids, and beta-blockers), biotin supplements (assay interference), and pregnancy can shift the components.
Best read with its components, free T3 and free T4, plus TSH and, where relevant, thyroid antibodies.
What does a high or low fT3/fT4 ratio mean? High suggests more T3 relative to T4; low suggests less conversion to T3. Use it with TSH, free T4, and free T3.
Do I need to fast for this test? No. For consistency, take thyroid medicines after your blood draw unless told otherwise.
What can affect my results? Biotin, thyroid dose timing, acute illness, intense exercise, and medicines like amiodarone or steroids can shift the ratio.
How often should I test? Only as advised by your clinician. When adjusting therapy, testing is typically spaced weeks apart to see trends.
How long do results take? Results are usually ready in about 7 days.
What should I discuss with my clinician? Share symptoms, medication list and timing, supplements, recent illnesses, and any pregnancy plans to guide interpretation.
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