The risk of thyroid dysfunction increases with age and is most prevalent in elderly females. Thyroid dysfunction often escapes clinical detection because its symptoms often mimic those changes associated with aging itself. Hypothyroidism can be masked by the clinical features which it shares with the symptoms of aging that include a general slowing of mental and physical function, tendency to low body temperatures and cold intolerance, weight gain, constipation, hardening of the arteries, elevation of serum lipids (cholesterol), elevation of blood pressure and anemia. Apathetic (indifferent) behaviour may be part of aging but may also be the presenting finding of hyperthyroidism in the elderly. Hyperthyroidism, with its associated irregular heart rhythms, congestive heart failure, nervousness, sweating, weight loss and muscle weakness, may also be misinterpreted as part of the aging process.
Owing to the atypical, non-specific and meagre clinical findings that may occur for thyroid disease among the elderly, appropriate recognition depends upon a high index of suspicion on the part of the clinician and confirmation by appropriate laboratory testing. However, the interpretation of thyroid tests requires a familiarity with the minimal reductions in circulating levels of thyroid hormones associated with aging, itself. In addition, co-existing decreases in caloric intake, acute and chronic nonthyroidal illnesses as well as a variety of pharmacologic agents and x-ray dyes also affect their interpretation. Such conditions may modify thyroid tests to either mask existing thyroid dysfunction or induce changes which simulate abnormal results by spuriously increasing or decreasing circulating levels of thyroid hormone concentrations [(thyroxine T4) or L-triiodothyronine (T3)] as well as levels of pituitary thyroid stimulating hormone (TSH). When these complicating factors are excluded, the usual normal ranges for younger adults with slight modifications for age should suffice in diagnosing both hypothyroidism and hyperthyroidism. In particular, newly devised methods for measuring serum TSH by ultrasensitive methods have improved the early detection of thyroid dysfunction in the elderly, especially when the above mentioned complicating factors have been excluded.
Detecting Hypothyroidism in the Elderly
When hypothyroidism is suspected in the elderly, a serum T4 and T3 Uptake (T3U) assay should be performed in conjunction with a routine measurement of serum TSH. In typical primary hypothyroidism, the T4 and T3U tests are below normal and the serum TSH increased. Routine screening of elderly populations (greater than 60 years of age) not uncommonly detects increases in serum TSH among approximately 15-20% of such subjects, with females having a 3-4 fold greater risk compared to males. However, the majority of elderly subjects detected by TSH screening are relatively asymptomatic and have a serum TSH level at less than 10 mIU/L ( with upper limit of normal being 3.5 mIU/L). It is estimated that among such patients with moderate increases in TSH, only 50% will have symptomatic improvement following a cautious trial of thyroid hormone replacement therapy; whereas, those who are more severely affected, (serum TSH levels usually greater than 20 mIU/L) often have a dramatic improvement in mental and physical function following appropriate treatment.
Treating Hypothyroidism in the Elderly
In the treatment of hypothyroidism in the elderly, those patients with minimal elevations of serum TSH but normal serum T4 and T3U levels, may not require immediate treatment unless there is a large goitre. Therapy may also be withheld when there are other complicating conditions such as heart disease. In general, the institution of thyroid hormone replacement therapy commences with L-thyroxine (L-T4) tablets at small initial doses of between 12.5 to 25 micrograms/day. The L-T4 dosage is increased at six weeks intervals to a maximum daily dose of approximately 75-125 micrograms/day to avoid complications of excessive replacement therapy such as nervousness, sweating, heat intolerance, weight loss, palpitation and chest pain. Usually, L-T4 is administered at a dosage sufficient to normalize serum T4 to mid-normal levels and restore serum TSH levels to the upper ranges of normal. However, in the elderly, the average replacement dosage is less than in younger adults particularly when there is co-existing cardiovascular disease. It has been well established that elderly patients with longstanding hypothyroidism that began in adulthood often will require a reduction of the daily maintenance dosage in later life. In particular, postmenopausal females receiving L-T4 dosage that suppresses the TSH level, could be at risk for increasing the degree of reduced bone density (osteoporosis) that commonly occurs in the elderly.
Detecting Hyperthyroidism in the Elderly
In the absence of possible coexisting systemic illness and pharmacologic drug interference, a low or suppressed TSH level indicates hyperthyroidism. Serum total T4 and T3U are routinely measured to determine the degree of hyperthyroidism. As well, serum T3 test is often obtained to detect a subtle type of hyperthyroidism known as the "T3 toxicosis syndrome" which may occur in elderly subjects who have either single or multiple hyperfunctioning thyroid nodules. When a diagnosis of hyperthyroidism is confirmed by clinical and laboratory evaluation, a radioisotope thyroid uptake and scan test should be subsequently performed to determine which of the various causes of hyperthyroidism is present, in order to assist in selecting the appropriate treatment.
Treatment of Hyperthyroidism in the Elderly
Most commonly hyperthyroidism in the elderly is caused by the overproduction of thyroid hormone within the thyroid gland due to either a diffusely overactive thyroid gland (Graves' disease) or, alternatively, by a hyperfunctioning solitary or multiple thyroid nodules or lump (i.e toxic nodular goitre). Such causes of hyperthyroidism in the elderly are optimally treated by the initiation of antithyroid medication (propylthioracil or methimazole) to block the thyroid hormone production. Also, sedatives and beta blocker medication (to reduce nervousness, tremor, sweating, and rapid heart rate) may be used with caution in the elderly to non-specifically control hyperthyroid symptoms until the antithyroid drugs restore the patient to normal. Once thyroid function has been normalized for three to six months on such medication, a more definitive therapy is usually selected to permanently relieve excessive thyroid gland function. Most commonly, radioactive iodine is given after discontinuing thionamide drugs for 7-14 days prior to its administration. When Graves' disease is treated with radioactive iodine, hypothyroidism often occurs but is less commonly observed when radioiodine is used for hyperfunctioning thyroid nodules. Should hypothyroidism occur, appropriate L-T4 replacement therapy is subsequently instituted. When there is a concern that hyperthyroidism coexists with a hypofunctioning thyroid nodule, (and the consequent risk for underlying malignancy) surgery may be selected as definitive therapy to deal with both problems simultaneously. After appropriate surgery, long-term L-thyroxine replacement therapy will be indicated if there is subsequent hypothyroidism.
Hyperthyroidism in the elderly may also be secondary to a destructive inflammatory process in the thyroid gland associated with pain (typical subacute thyroiditis - de Quervain's syndrome). Subacute thyroiditis is believed to be secondary to an infection (often due to a viral cause). Alternatively, a painless thyroiditis (underlying Hashimoto's thyroiditis) can also cause hyperthyroidism. Such hyperthyroidism may be suspected by its association with a suppressed radioactive iodine thyroid uptake test. The hyperthyroidism spontaneously resolves over weeks to months. These causes of hyperthyroidism only require therapy directed at the temporary symptoms and signs of hyperthyroidism by using sedatives and/or beta blocker drugs until the hyperthyroidism has resolved spontaneously. However, when the inflammation is severe, there is risk 2-4 months later of transient hypothyroidism that may require L-thyroxine replacement therapy for 6-12 months until thyroid gland function recovers.
Summary
Since it may mimic the symptoms and signs of aging, the clinical recognition of thyroid dysfunction in the elderly requires a high index of suspicion. Although the standard tests used to monitor thyroid function may be slightly altered by aging, the "normal ranges" used for younger adults can be applied for diagnosing thyroid dysfunction in the elderly when factors of concurrent acute or chronic illness, changes in nutritional and mental status as well as the possible recent administration of x-ray dyes and drugs have been excluded.
Treatment of thyroid dysfunction in the elderly compared to younger patients, does require special considerations. Generally, L-thyroxine therapy for the treatment of hypothyroidism is initiated at low dosage and the eventual long term maintenance dose required to restore elderly subjects to normal is less than that necessary for younger patients. The treatment of hyperthyroidism in the elderly requires the routine restoration of thyroid function to normal for 3-6 months using antithyroid drugs (propylthioracil or methimazole) prior to administering either radioactive iodine or surgery to permanently eliminate the possibility of recurrent hyperthyroidism.
Written by Paul G. Walfish, CM, MD, FRCP(C), FACP, FRSM, Professor of Medicine & Pediatrics, University of Toronto, Mount Sinai Hospital, Toronto.