Triiodothyronine (T(3)) is the major thyroid hormone and thyroxine (T(4)) may be only a "prohormone". A normal serum T(3) concentration can compensate for a low serum T(4) to maintain euthyroidism and on the other hand hyperthyroidism can exist in spite of a normal T(4) if the T(3) concentration is increased ("T(3)-toxicosis"). A raised serum thyroid stimulating hormone (TSH) concentration is the present most sensitive indicator of thyroidal hypothyroidism and the level can be used to titrate replacement therapy to the individual's own requirements. TSH concentration is classically low in hypothyroidism secondary to pituitary or to hypothalamic disorder and synthetic thyrotrophin release hormone can then be used to identify which of these two sites is at fault. Thyroxine is the best form of thyroid replacement for hypothyroidism because it produces more consistently physiological concentrations of T(3). Full replacement is achieved with 0.1 - 0.2 mg of T(4)/day and doses above this, as formerly widely used, may cause over-replacement. New reliable kit tests are available which give in one quick procedure a measure of free-thyroxine even in the presence of abnormalities of protein-binding. These kit tests are suitable for the routine screening of the whole spectrum of thyroid dysfunction and when combined, in appropriate instances, with radioimmunoassay procedures for serum T(3) and TSH, provide a battery of tests which will help in the diagnosis of the great majority of causes of thyroid dysfunction.