fT4 / TSH ratio

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.

Because TSH is profoundly suppressed in Graves' disease, this ratio can run far higher than in subacute thyroiditis, so it has been proposed as a rapid way to tell the two apart when radioactive iodine uptake testing or antibody panels aren't available or are contraindicated (e.g. pregnancy). However, the exact cutoff depends heavily on each lab's lowest detectable TSH limit, so there is no standardized, officially established threshold for this ratio — it is directional information only.

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.