Sheldon Singh, BSc

Hypothyroidism (thyroid hormone deficiency) can affect all body systems; the degree of severity can range from mild and unrecognized to a state of striking myxedema. In the elderly, hypothyroidism is not unusual. However, it may go unrecognized for a variety of reasons including mistaking the non-specific signs and symptoms as a part of the "normal aging" process or other medical conditions that may exist in the elderly. This article will discuss normal thyroid physiology, changes to the thyroid as a result of the aging process, etiology of hypothyroidism as well as diagnosis, management and screening for hypothyroidism.

Physiology of the Thyroid
A review of the physiology of the normal feedback loop of the hypothalamic-pituitary-thyroid axis is quite useful in aiding clinicians in interpreting thyroid function tests and subsequently diagnosing thyroid dysfunction (see figures 1 and 2). Thyrotropin-releasing hormone (TRH) is produced by the hypothalamus. This hormone stimulates thyrotroph cells in the anterior pituitary to secrete thyrotropin (TSH). This hormone then stimulates the thyroid follicular cells to release tri-iodothyronine (T3) and thyroxine (T4). The release of T3 and T4 suppress the release of TSH and TRH; this completes the feedback loop. Although T3 differs from T4 by one iodide molecule (see figure 1 on page 18), this difference results in a significant increase in receptor affinity and activation.