Conversion and assimilation of thyroid hormones

[hellohelenhere]

What happens if the problem is not with production of T4, but with conversion of T4 to T3, or with assimilation of T3 into the cells? Could you please explain how you would detect and treat in these cases?

[Dr Graham Beastall]

A primary cause of reduced conversion of T4 to T3 is extremely rare. Likewise receptor defects that alter the uptake of thyroid hormone into the cell are extremely rare.

In contrast intercurrent illness of a non-thyroidal nature can bring about subtle effects on thyroid hormone metabolism. The best illustration is in elective surgery when the conversion of T4 into T3 reduces for a few days before returning to normal as the patient recovers. This is a normal and necessary physiological response to surgery and does not require treatment. Thyroid function tests should not be performed in these circumstances.

In chronic illness it is more complex, especially when the primary cause of that chronic illness cannot be identified and treated. In such circumstances thyroid function tests are likely to be within normal reference ranges because the effect is subtle - it it exists at all in the individual patient. Very few bona fide clinical trials of T3 replacement have been carried in these chronic disease situations and those that have been performed have not produced clear cut results.