Thyroid hormones and thyroid reserve in preclinical hypothyroidism

Preclinical hypothyroidism (i.e. basal thyroxine within the normal laboratory range, basal TSH normal or elevated and exaggerated TSH response to TRH) is a biochemical constellation of uncertain clinical relevance. The oral TRH test with simultaneous measurements of TSH and thyroid hormones before and 3 h after 40 mg TRH provides information about both pituitary and thyroid reserve. In a group of female patients with preclinical hypothyroidism, basal thyroxine but not triiodothyronine was found to be clearly diminished compared with a group of healthy female controls, indicating a slight thyroid hormone deficiency. Furthermore, a progressively reduced thyroid reserve of T4 and especially of T3 was seen to be closely related to elevation of basal TSH as an expression of thyroid cell insufficiency. These data emphasize the clinical importance of TSH elevation despite normal thyroxine levels as a better individual sign of impending primary hypothyroidism.