[Markers of thyroid function (I). Assessment of glandular activity]

Thyroid hormones have a fundamental role in the metabolism, and are active in all organs and systems. Thyroid dysfunction (TD) is an important public health problem that concerns 10% of the general population. Diagnosis of TD is currently carried out on the basis of serum hormone levels determined by laboratory analysis. The results are classified into clinical (TSH and thyroid hormones out of normal range) and subclinical (isolated TSH disorder) dysfunction. The development of third generation assays in the TSH test represents a great step forward in the diagnosis of TD. However, the debate about the pathological significance of subclinical TD has intensified. There is as yet no agreement on the best approach for diagnosis and treatment of this common condition. It is currently impossible to know which subjects with subclinical TD will develop the typical complications of clinical dysfunction. In this setting, it is feasible that a number of subclinical hypothyroid subjects may in some cases be overtreated with thyroxin, while others remain undertreated with increased iatrogenic morbidity as a consequence.