Thyroid function in the elderly.

Thyroid dysfunction is common in older individuals, yet the diagnosis is often complicated by atypical clinical presentations and difficulty in interpretation of laboratory tests. An understanding of the alterations in thyroid function occurring normally as a consequence of the aging process is necessary for correct laboratory diagnosis of thyroid dysfunction in the elderly. There are subtle alterations in hypothalamic and pituitary function but normal feedback control of TSH secretion persists. In the thyroid itself, morphologic changes develop with age, but have little impact on thyroid hormone economy. Thyroidal secretion of thyroxine decreases, but parallels the decrease in thyroxine degradation rate, resulting in unaltered plasma thyroxine levels. Decreased peripheral conversion of thyroxine to triiodothyronine causes a fall in triiodothyronine concentrations. Nonthyroidal illnesses in the elderly may perturb the laboratory assessment of thyroid function by producing isolated high or low thyroxine levels in euthyroid individuals.