Thyroid disorders in the elderly population are common, often challenging diagnostically, and frequently overlooked. The clinical presentations of thyroid diseases maybe subtle, with nonspecific signs and symptoms that are attributed to other illnesses or to a normal aging process. Thyroid function tests can be misleading in the presence of concurrent acute or chronic diseases and may be affected by some medications. This chapter describes the most common thyroid disorders encountered in the elderly population.
The thyroid gland is the largest endocrine organ in the human body, and weighs approximately 12 to 20 g in adults. The structural and functional changes of the thyroid gland that occur with aging are controversial. Some investigators report that there were no size or weight changes, others found increases to twice normal size after age 70 years, whereas other reports indicated that the thyroid gland undergoes atrophy, fibrosis, and decrease in weight. The thyroid gland is also more nodular with advancing age, and there is an increase in fibrosis and lymphocytic infiltration. Despite these changes, normal thyroid function is maintained by the vast majority of the elderly population. Estimation of the thyroid size and its palpation may be difficult in elderly patients because of cervical kyphosis, obesity, or chronic pulmonary disease.
Iodine, an essential substrate for synthesis of thyroid hormone, is absorbed from the diet and enters the circulation as inorganic iodide that is distributed in extracellular fluids as well as in salivary, breast, and gastric secretions. The average daily iodine intake is about 250 μg/day in the United States. A 24-hour urinary iodine measurement is an index of dietary iodine intake.
Iodide is actively concentrated by the thyroid gland or cleared from the plasma by the kidney. The thyroid gland, compared with the kidneys, is the active participant in the competition for plasma iodide and adjusts the rate of entry of iodide into the thyroid tissue based on the changes in thyroid hormone synthesis rather than renal avidity for iodide ion. The active transport of iodide from plasma to follicular cell is carried out by the sodium iodide symporter, a transport protein on the follicular cell plasma membrane. The extracellular fluid iodide concentration is usually very low because of the rapid clearance of iodide from extracellular fluid by the thyroidal uptake and renal clearance.
Table 108-1 summarizes the aging effects on thyroid function. The renal and thyroidal iodide clearance rate diminishes with advancing age. Thyroid iodide clearance, estimated by a 24-hour radioactive iodine uptake by the thyroid gland, decreases in euthyroid subjects after age 60 years. Urinary iodine excretion also was found to be significantly reduced in subjects older than 80 years of age.
Table 108-1 Age-Related Changes in Thyroid Physiology