The ratio of free T4 to TSH, a summary of thyroid balance.
The fT4/TSH ratio is a calculated value, not a separate blood test. It is free T4 divided by TSH (thyroid-stimulating hormone). It combines the main thyroid hormone with the pituitary signal that controls it.
Normally, TSH and free T4 move in opposite directions: when thyroid hormone is low, TSH rises to push the thyroid harder. The ratio captures this relationship in one number and has been studied to help judge thyroid status and, in some research, pituitary function. It is read alongside the standard thyroid panel, not in isolation.
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, and it is sensitive to the units and assays used. It is interpreted alongside the individual thyroid values rather than as a standalone threshold.
| Pattern | Interpretation |
|---|---|
| Lower ratio | High TSH relative to free T4; suggests underactive thyroid |
| Higher ratio | Low TSH relative to free T4; suggests overactive thyroid |
Thresholds are not standardised. Source: American Thyroid Association, thyroid function tests.
TSH varies through the day and rises with poor sleep and stress. Acute illness, pregnancy, biotin supplements (assay interference), and several medicines can shift either component, which changes the ratio. The relationship breaks down in pituitary disease.
Best read with its components, free T4 and TSH, plus free T3 and thyroid antibodies where relevant.
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