We've a new paper out, in which we measure the contribution of T4 and T3 in controlling TSH levels. T4 is slightly stronger in this than T3 but not by much. Also we've a first estimate of how much on average the thyroid directly contributes to T3 production overall. The first answer seems to be around 1/4 of the total production. That is, the body provides the other 3/4 by T4-T3 conversion. Therefore in patients with no thyroid the body has to work extra hard to produce the missing T3, needing more T4 to achieve it (even if the body can which for some people it can't) and therefore suppressing TSH even down to undetectable levels in some cases. Ergo, the "healthy" TSH level is inappropriate as a diagnostic for patients on T4 with little or no working thyroid.
Title -Dual control of pituitary thyroid stimulating hormone secretion by thyroxine and triiodothyronine in athyreotic patients
Journal- Therapeutic advances in endocrinology and metabolism 8(6):83-95 · July 2017