4 Investigation and treatment of hypothyroidism

Abstract

INVESTIGATION The investigation of patients with suspected hypothyroidism falls naturally into two stages. Preliminary investigation is required to determine the level of thyroid function and confirm, or refute, a clinical diagnosis of thyroid failure, and the use of other diagnostic procedures is required to identify the cause of hypothyroidism in some patients. Tests to Determine the Level of Thyroid Function The laboratory techniques available for assessing the level of thyroid function fall into three groups and the use of each allows different aspects of impaired thyroid function to be defined. The three groups of techniques are: tests of thyroid function; tests of peripheral tissue function; and tests of hypothalamic--pituitary function. The techniques have been reviewed in detail elsewhere (Evered, 1974, 1976). Tests of thyroid function Direct or indirect techniques for measuring circulating thyroid hormone concentrations have been widely available for many years. Serum thyroxine (T4) concentration may be measured by a competitive protein binding assay (Murphy, 1965) or by radioimmunoassay (Mitsuma et al, 1972). These procedures measure total serum T4 concentration, and thus alterations in thyroxine-binding globulin (TBG) capacity (e.g. pregnancy, oral contraceptive therapy, liver disease, etc.) may lead to total hormone concentrations which are at variance with the clinical assessment of the patient. The serum T4 concentration can, however, be corrected for changes in TBG by carrying out a thyroid hormone binding test (e.g. T3 resin uptake, T3 Sephadex uptake), which will estimate residual TBG capacity and may be used to calculate a free thyroxine index (FTI) (Clark and Horn, 1965). The older technique of estimating the protein-bound iodine (PBI) concentration also