Introduction

The prevalence of hypothyroidism and thyroid nodules increases dramatically in elderly persons. Although the prevalence of hyperthyroidism is similar for younger and elderly patients, its presentation in elderly patients may be nonspecific and it may be difficult to diagnose.

Thyroid Function in Normal Aging

The thyroid gland produces thyroxine (T₄) and triiodothyronine (T₃). With age, the thyroid gland undergoes moderate atrophy and develops nonspecific histopathologic abnormalities: fibrosis, increasing numbers of colloid nodules, and some lymphocytic infiltration. Production of T₄ decreases by about 30% between young adulthood and advanced age. However, because serum T₄ levels remain unchanged with age, the decrease in T₄ is considered to be physiologic compensation for decreased use of the hormone by tissue and not a manifestation of primary thyroid failure.

The body's decrease in use of T₄ correlates with the age-related decline in lean body mass, suggesting that the mass of metabolically active, protein-rich tissue (ie, muscle, skin, bone, and viscera) decreases, which may lead to reduced use and catabolism of thyroid hormones. Thyroid hormone levels rise subtly, and thyroid-stimulating hormone (TSH) output decreases. T₃ and T₄ output decreases, and serum T₄ levels return to normal. When stimulated by increased TSH, the healthy aged thyroid gland can increase its hormone production normally.

Serum T₃ and free T₃ levels decrease moderately with age. This decline is thought to be due to a combination of decreased monodeiodination of the outer ring of T₄ and decreased pituitary secretion of TSH. In healthy centenarians, the decline in serum T₃ may be as much as 35%.

The average serum TSH level rises with age. Although this increase appears to contradict the explanation given for the constancy of serum T₄, it probably reflects the high prevalence of Hashimoto's disease leading to hypothyroidism in elderly patients. Hashimoto's disease (also called chronic autoimmune thyroiditis) is an autoimmune inflammatory disease of the thyroid gland. The increase in serum TSH level correlates with elevated antibody levels and disappears when patients with Hashimoto's disease are excluded from consideration.