Hypothyroidism: Clinical features and complications

Abstract

Hypothyroidism may be defined as the clinical state which results from decreased production of thyroid hormones, or, very rarely, from their decreased action at a tissue level. Virtually every tissue in the body is affected to a greater or lesser extent by thyroid hormone deficiency. The clinical picture which results depends on a variety of factors -- the rate of onset of the deficiency and its severity. Though there are great individual differences in response to diminished levels of thyroid hormone, gross deficiency may sometimes be associated with only minor clinical features of the disease. Hypothyroidism occurs most frequently in women, the sex ratio of symptomatic hypothyroidism being about 10:1. Although the disease can occur at any age, most patients present between 30 and 60 years. The great majority of patients who present with hypothyroidism are suffering from primary thyroid disease. Pituitary hypothyroidism is rare and will not be considered in this chapter. It should be suspected if there is other clinical evidence of primary pituitary disease, e.g. acromegaly, Cushing's syndrome or some visual defect. The presence of circulating thyroid autoantibodies, found in the majority of patients with thyroid failure, is usually accepted, along with elevation of the serum TSH, as sufficient evidence for primary thyroid disease. The clinical syndrome of hypothyroidism was first described by Gull in 1874. The term 'myxoedema' was coined by Ord four years later, and, although Haliburton pointed out in 1893 that 'myxoedema' was not a constant feature of hypothyroidism, the terms were considered to be synonymous for many years. It was widely accepted during the early years of this century that hypothyroidism was an 'all or none' phenomenon. However, the advent of more precise diagnostic techniques has demonstrated that hypothyroidism is a graded phenomenon (Evered and Hall, 1972). The term 'myxoedema' should be reserved for that particular presentation of advanced hypothyroidism in which there is undue swelling of the skin and subcutaneous tissues.