[Markers of thyroid function (II). Assessment of tissue action]

Advances in molecular medicine have increased our knowledge of the consequences of hormone action in target cells. We are currently able to determine to some extent the molecular thyroid hormone activity in different organs. The effects are related with a variety of factors, but their association with plasmatic hormone levels is only partially correlated. Recent advances indicate that there are several intermediate factors in thyroid tissue activity. The iodothyronine selenodeiodinases have a relevant role in this context. The clinical and biochemical methods currently available for thyroid function assessment do not permit us to explore many of these new elements. However, it is well known that thyroid hormones enhance the expression of a number of proteins, and some of these can be measured by simple methods. Accordingly, the plasmatic value of these proteins may be related with the effect of thyroid hormones in the target tissues, which is the result of thyroid function. The ability to determine the tissue activity of thyroid hormones will enable us to administrate the treatment dose more accurately, only to patients who require it, avoiding iatrogenism.