I was looking through some old papers on the use of thyroid hormone tests in patients on T4 therapy and I came across this from 1986 which TUK might find interesting.
It's in: Free thyroid hormone assays and thyroid function: CJ Pearce and PGH Byfield; Ann Clin Biochem 23: 230-237: 1986.
In health the thyroid gland secretes both T4 and T3, but the major contribution to the latter's daily production rate is via the peripheral metabolism of T4 . lf the thyroid gland is absent as a result of disease, surgery or radioiodine and if thyroxine replacement is given, then the patient is entirely dependent on the extrathyroidal conversion of T4 for T3 production. Because thyroidal input of T3 is absent it may be necessary to achieve higher serum T4 levels than 'normal' to provide an adequate level of T3. A significant proportion of healthy patients stabilised on long-term replacement therapy for hypothyroidism have both total and free T4 concentrations above the normal range. In a third of such patients both free and total T4 were elevated, while total and free T3 remained normal in the great majority. It was suggested that the finding of an elevated total or free T4 did not necessarily imply over-treatment, and that a more helpful investigation would be the demonstration of a normal free T3 in the presence of an adequately suppressed TSH concentration, the latter representing adequacy of replacement. In this series the same information was derived from the total serum T3, as none of these subjects had an abnormal TBG concentration.
Does this ring a bell, and what does it say about the wisdom of 30 years ago? Our group really is only repeating in more detail what was said then.