Anesthesia for Hypothyroidism

HYPOTHYROIDISM

Clinical Manifestations

Hypothyroidism can be caused by autoimmune dis-ease (eg, Hashimoto’s thyroiditis), thyroidectomy, radioactive iodine, antithyroid medications, iodine deficiency, or failure of the hypothalamic–pituitary axis (secondary hypothyroidism). Hypothyroidism during neonatal development results in cretinism, a condition marked by physical and mental retarda-tion. Clinical manifestations of hypothyroidism in the adult are usually subtle and include infertility, weight gain, cold intolerance, muscle fatigue, leth-argy, constipation, hypoactive reflexes, dull facial expression, and depression. Heart rate, myocar-dial contractility, stroke volume, and cardiac out-put decrease, and extremities are cool and mottled because of peripheral vasoconstriction. Pleural, abdominal, and pericardial effusions are common. Hypothyroidism may be diagnosed by an elevated TSH concentration, or a reduced free (or total) T ₃ level, or both. Primary hypothyroidism, the more common condition, is differentiated from secondary disease by an elevation in TSH in the former. Normal concentrations of TSH despite reduced T₃ concentra-tions (the so-called “euthyroid sick” syndrome) are often seen in critical illness. The treatment of hypo-thyroidism consists of oral replacement therapy with a thyroid hormone preparation, which takes several days to produce a physiological effect and several weeks to evoke clear-cut clinical improvement.

Myxedema coma results from extreme hypothy-roidism and is characterized by impaired mentation, hypoventilation, hypothermia, hyponatremia (from inappropriate antidiuretic hormone secretion), and congestive heart failure. It is more common in elderly patients and may be precipitated by infection, surgery, or trauma. Myxedema coma is a life-threat-ening disease that can be treated with intravenous T₃. T₄ should not be used in this circumstance to avoid the need for peripheral conversion to T₃. The ECG should be monitored during therapy to detect myo-cardial ischemia or arrhythmias. Steroid replacement (eg, hydrocortisone, 100 mg intravenously every 8 h)is routinely given due to frequent coexisting adrenal gland suppression. Some patients may require venti-latory support and external warming.

Anesthetic Considerations

A. Preoperative

Patients with uncorrected severe hypothyroidism or myxedema coma should not undergo elective sur-gery. Such patients should be treated with T₃ intra-venously prior to emergency surgery. Although a euthyroid state is ideal, mild to moderate hypothy-roidism does not appear to be an absolute contra-indication to surgery, for example, urgent coronary bypass surgery.

Hypothyroid patients usually require minimal preoperative sedation and are very prone to drug-induced respiratory depression. In addition, they may fail to respond to hypoxia with increased min-ute ventilation. Patients who have been rendered euthyroid may receive their usual dose of thyroid medication on the morning of surgery; it must be remembered, however, that most commonly used preparations have long half-lives (the half-life of T ₄ is about 8 days); therefore, omission of a single dose should have no medical importance.

B. Intraoperative

Clinically hypothyroid patients are more sus-ceptible to the hypotensive effect of anestheticagents because of their diminished cardiac output, blunted baroreceptor reflexes, and decreased intra-vascular volume. For these reasons, ketamine or etomidate can be recommended for induction of anesthesia. The possibility of coexistent primary adrenal insufficiency should be considered in cases of refractory hypotension. Other potential coex-isting conditions include hypoglycemia, ane-mia, hyponatremia, difficulty during intubation because of a large tongue, and hypothermia from a low basal metabolic rate.

C. Postoperative

Recovery from general anesthesia may be delayed in hypothyroid patients by hypothermia, respiratory depression, or slowed drug biotransformation; thus these patients may require mechanical ventilation. Because hypothyroidism increases vulnerability to respiratory depression, a multimodal approach to postoperative pain management, rather than strict reliance on opioids would be appropriate.