T3 and T4 are particularly important for normal growth of the skeleton and of the central nervous system. T3 and T4 have important metabolic effects, increasing the breakdown and use of carbohydrates and lipids by the body, and determining to a large extent the basal metabolic rate and the level of heat production. An underactive thyroid takes up the isotope dose more slowly, whereas an overactive thyroid takes it up more quickly. Cretinism is associated with retarded growth, sluggish movements and severe mental deficiencies. The growth deficiencies can be corrected by providing thyroid extract or iodine supplementation soon after birth, but mental impairment tends to be irreversible.
Adults suffering from hypothyroidism show a very low rate of metabolism and are extremely lethargic.
Body temperature, heart rate and respiratory rate are all low. The patient is prone to constipation and has a poor appetite. They are mentally slow and often adopt a vacant expression, and show an absence of libido. Hypothyroidism is also associated with a number of changes to the skin and subcutaneous tissues. The outer surface of the skin becomes scaly due to deterioration of sebaceous glands, and oedematous swelling occurs in the subcutaneous connective tissue, this is particularly noticeable around the face.