fT4 / TSH ratio
Stomach and gut healthAlso known as: free T4 to TSH ratio, thyroid ratio
The ratio of free thyroxine (fT4) to thyroid-stimulating hormone (TSH) provides additional insight into thyroid function by evaluating how the pituitary gland responds to circulating thyroid hormone levels.
It can reveal subtle thyroid dysfunction that individual fT4 or TSH values alone may not capture.
Why this matters
The ratio enhances early detection of thyroid dysfunction and helps assess thyroid reserve capacity. A declining ratio may indicate developing hypothyroidism before individual values become abnormal, while an elevated ratio can suggest hyperthyroidism or excellent thyroid responsiveness. It's particularly useful for detecting subclinical thyroid disease, monitoring treatment effectiveness, and identifying central (pituitary) versus primary (thyroid) dysfunction.
How this connects to other biomarkers
- fT4/TSH ratio integrates the brain-thyroid feedback loop — a low ratio (high TSH, low fT4) confirms underactive thyroid (primary hypothyroidism), while a high ratio (suppressed TSH, high fT4) confirms overactive thyroid (primary hyperthyroidism).
- In brain/pituitary thyroid disorders, the ratio can be misleadingly normal because TSH is inappropriately low or normal despite low fT4.
How often should I test fT4 / TSH ratio?
The fT4/TSH ratio is reassessed alongside the full thyroid panel, yearly for most adults, or 6 to 8 weeks after starting or adjusting thyroid medication.
At baseline / for screening: Once every 12 months from age 30. More frequently if you have a family history of thyroid disease, are pregnant or planning pregnancy, have an autoimmune condition, or notice symptoms suggesting thyroid dysfunction (energy, weight, mood, temperature regulation).
When monitoring an intervention or change: Retest 6 to 8 weeks after starting or adjusting thyroid medication (levothyroxine, liothyronine, antithyroid drugs), since that's how long TSH takes to reach a new steady state. Acute illness and severe caloric restriction can shift free T3 downward independent of thyroid function, so wait for those to resolve before reading the trend.
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