[Hypothyroidism--clinical findings, diagnosis, therapy. Thyroid tests should be performed on broad indications]

Hypothyroidism is a clinical entity resulting from deficiency of thyroid hormones or, more rarely, from their impaired activity at tissue level. It is a common condition, with a prevalence of 1.9 per cent in women, and the prevalence increases with age. Hypothyroidism may be congenital or acquired, primary or secondary, chronic or transient. Primary hypothyroidism is caused by disease or treatment which destroys the thyroid gland or interferes with thyroid hormone biosynthesis. Autoimmune thyroiditis is the predominant cause of primary hypothyroidism in countries such as Sweden where severe iodine deficiency is non-existent. Another cause of primary hypothyroidism, chronic or transient, is previous radio-iodine or surgical treatment of hypothyroidism. In secondary or central hypothyroidism, which is very rare, there is a lack of thyroid-stimulating hormone (TSH) or TSH activity, due to a pituitary or hypothalamic cause. The clinical features of hypothyroidism are dependent on the patient's age, the presence of other disease, and the rate at which hypothyroidism develops. As thyroid hormones are universal determinants of organ function, there may be a multiplicity of symptoms. Particularly in the elderly, the clinical features may be atypical, and the diagnosis easily missed. First line tests for hypothyroidism are analyses of the concentrations of free thyroxine (T4) and TSH in serum. In primary hypothyroidism, the serum content of T4 is low and that of TSH high. In central hypothyroidism, the serum content of T4 is low and that of TSH generally low or normal, though slightly increased levels of biologically inactive TSH may also occur. Subclinical hypothyroidism is characterised by a normal serum level of T4, an increased level of TSH, and the absence of clinical symptoms. When a diagnosis of chronic hypothyroidism is confirmed, treatment with laevothyroxine is started, the initial dose being adjusted to the age and general condition of the patient, and the duration and severity of hypothyroidism. As a rule, full thyroxine replacement therapy should bring the serum TSH level into the normal range. In central hypothyroidism, laevothyroxine treatment is similar, but pituitary function must be evaluated and, if necessary, corticosteroid replacement be instituted before laevothyroxine treatment is started.