Nondiabetic endocrine disease

1

Review the thyroid hormone laboratory tests.

Thyroid hormone laboratory tests include:

• Total thyroxine (T ₄ ) level

• Total triiodothyronine (T ₃ ) level—formed from the peripheral conversion of T ₄

• Thyroid-stimulating hormone (TSH) level—formed in anterior pituitary

• Resin T ₃ uptake (T ₃ RU). T ₃ RU is useful in conditions that alter levels of thyroid-binding globulin, which would alter total T ₄ results ( Table 48.1 )

  Table 48.1

  Usefulness of Thyroid Function Tests in the Diagnosis of Hypothyroid or Hyperthyroid States

  Disease	T 4	T 3	TSH	T 3 RU
  Primary hypothyroidism	−	−	+	−
  Secondary hypothyroidism	−	−	−	−
  Hyperthyroidism	+	+	0	+
  Pregnancy	+	0	0	+

  T ₄ , Thyroxine; T ₃ , tri-iodothyronine; TSH , thyroid-stimulating hormone; T ₃ RU , resin T ₃ uptake; +, increased; −, decreased; 0 , no change.

• Thyrotropin-releasing hormone (TRH)—produced by the hypothalamus

2

What are common signs and symptoms of hypothyroidism?

• Symptoms include fatigue, cold intolerance, constipation, dry skin, hair loss, weight gain.

• Signs include bradycardia, hypothermia, decreased tendon reflexes, hoarseness, periorbital edema.

• Long-term, untreated hypothyroidism may progress to myxedema coma, which can be fatal. Myxedema is characterized by hypoventilation, hypothermia, hypotension, hyponatremia, and hypoglycemia (the “hypos”), as well as obtundation and adrenal insufficiency. Note, Addisonian crisis can have a similar presentation to hypothyroidism.

3

Review the causes of hypothyroidism.

The most common causes of hypothyroidism include surgical or radioiodine ablation of thyroid tissue during the treatment of hyperthyroidism and, most commonly, Graves disease. Other causes of hypothyroidism include chronic thyroiditis (Hashimoto thyroiditis), drug effects, such as amiodarone and lithium, iodine deficiency, and pituitary or hypothalamic dysfunction. It can also be cause by infiltrative disorders, such as amyloidosis, sarcoidosis, hemochromatosis, and scleroderma.

4

Which manifestations of hypothyroidism have the greatest anesthetic consideration?

Hypothyroidism causes depression of myocardial function. Cardiac output declines as a result of decreased heart rate and stroke volume. Decreased blood volume, baroreceptor reflex dysfunction, and pericardial effusion may also accompany hypothyroidism. Conduction delays, symptomatic bradycardia and prolonged QT interval, leading to polymorphic ventricular tachycardia are also possible in severe hypothyroidism. Subsequently, the hypothyroid patient will be more sensitive to the hypotensive effects of anesthetics.

Hypoventilation may also be a feature of hypothyroidism. The ventilatory responses to both hypoxia and hypercarbia are impaired, making the hypothyroid patient sensitive to drugs that cause respiratory depression. Hypothyroidism also decreases the hepatic and renal clearance of drugs. Lastly, patients are prone to hypothermia because of lowered metabolic rate and consequently, lowered heat production.

5

How does hypothyroidism affect the minimum alveolar concentration of anesthetic agents?

Animal studies show that minimum alveolar concentration (MAC) is not affected by hypothyroidism. But in clinical cases, it has been noted that hypothyroid patients have increased sensitivity to anesthetic agents. This is caused not by a decrease in MAC per se, but by the patient's metabolically depressed condition.

6

Should elective surgery be delayed in a hypothyroid patient?

Patients with mild to moderate hypothyroidism are not at increased risk when undergoing elective surgical procedures. Some authorities suggest that elective surgery in patients who are symptomatic should be delayed until the patient is rendered euthyroid. In patients with severe hypothyroidism, elective surgery should be delayed until they have been rendered euthyroid. This may require 2 to 4 months of replacement therapy for complete reversal of the cardiopulmonary effects. Normalization of the patient's TSH level reflects reversal of hypothyroid-induced changes.

In the event of emergency surgery and severe hypothyroidism, administration of intravenous (IV) T ₃ /T ₄ is the appropriate measure. One should also consider administration of steroids, given high likelihood of adrenal insufficiency. While administering IV T ₃ /T ₄ , one must monitor for ST changes and myocardial ischemia.

7

What is the most common electrolyte deficiency in hypothyroidism?

Hyponatremia is the most common electrolyte abnormality seen in hypothyroidism. It is caused by the impaired free water excretion that results from renal impairment and excess vasopressin excretion. Extreme hyponatremia can affect mental status and should be corrected cautiously, as it can lead to cerebral osmotic demyelination syndrome.

8

List common signs and symptoms of hyperthyroidism.

• Symptoms include anxiety, tremor, heat intolerance, and fatigue.

• Signs include goiter, tachycardia, proptosis, atrial fibrillation, weight loss, and weakness.

• Causes of hyperthyroidism include Graves disease, thyroiditis, toxic multinodular goiter, and excessive iodine intake.

9

How is hyperthyroidism treated?

• Antithyroid drugs, such as propylthiouracil (PTU) inhibit iodination and coupling reactions in the thyroid gland, thus reducing production of T ₃ and T ₄ . PTU also inhibits peripheral conversion of T ₄ to T ₃ . Iodine in large doses not only blocks hormone production but also decreases the vascularity and size of the thyroid gland, making iodine useful in preparing hyperthyroid patients for thyroid surgery.

• Radioactive iodine, ¹³¹ I, is actively concentrated by the thyroid gland, resulting in destruction of thyroid cells and a decrease in the production of hormone.

• Surgical subtotal thyroidectomy