Introduction
	Hypothyroidism is a state which results from decreased production of thyroid hormones or, rarely, from their decreased action at tissue level. Hypothyroidism is a common disease, occurring most frequently in women. Thyroid hormones affect most tissues and manifestations of hypothyroidism are rather uniform between patients, although they depend on the intensity and the duration of the deficiency. Hypothyroidism and myxedema are considered to be synonymous. However, the term "myxedema” is appropriate only for the more advanced forms with swelling of the skin and subcutaneous tissue. Current views on hypothyroidism have benefited from several recent improvements and specific areas of interest have been delineated. Knowledge of the biology of thyroid hormone receptors is in rapid progress through the analysis of mutant receptors as well as the study of their regulation factors and also the analysis of the distribution of the various isoforms of the receptor in various tissues. Current research focuses also on the role of thyroid hormone in brain development at the earliest stages. The importance of this research is in line with the study of the neurological defects observed in areas of severe iodine deficiency as well as the recent observations of the potential role of maternal thyroid hormone in early fetal development. Finally, the increasing prevalence with age of subclinical hypothyroidism in the general population stresses the necessity of a better assessment of thyroid hormone effects at the level of the various functions and tissues.