As seen in Table 1, the clinical features of excess or deficient thyroid hormone are almost a mirror image of one another and reflect known actions of T3 in various target tissues. Depending on the cause of the condition, both hyper- and hypothyroid individuals often present with an enlarged thyroid gland, known as goiter, due to overstimulation of the TSH receptor.
Table1. Some Clinical Features of Disturbed Thyroid Hormone Status
In instances of thyroid hormone excess, the subject has an accelerated BMR and an elevated cardiac output associated with high body temperature, warm skin, and sweating that is inappropriate for the ambient temperature. Because muscle mass constitutes 50% of body weight, and because muscle tissue has a high rate of oxygen consumption, the increased muscular metabolic activity makes a substantial contribution to the higher BMR. Although hyperthyroid individuals often increase their food intake, there is usually a concomitant weight loss related to the greater gastrointestinal activity and associated diarrhea. Such an individual is often hyper active, with rapid movements and exaggerated reflexes, and frequently exhibits short attention span. Muscle myopathy occurs and is attributable to both abnormal protein catabolism with negative nitrogen balance and abnormal neuromuscular transmission.
In a hypothyroid subject, both BMR and cardiac output are reduced. Accordingly, the body temperature is lowered and the skin is cool and dry, since sweating is reduced in relation to the ambient temperature. The appetite of a hypothyroid individual is poor and there is reduced gastrointestinal activity. Although the skeletal muscles can be somewhat enlarged, there is no obvious myopathy. The principal external clinical features of hypothyroidism include a myxedemic appearance, described as an accumulation under the skin of mucoproteins and fluids resulting in a puffiness as a consequence of alterations in electrolyte and water balances. In addition to the features mentioned in Table1, a deficit of thyroid hormone during development and child hood results in impaired skeletal growth. The effects of T3 on the nervous system continue through adulthood so that the hypothyroid individual often shows signs of lethargy, and poor memory. In severe cases of long standing, a precipitating event, such as cold exposure, can result in myxedema coma.
