Thyroid Dysfunction

The major thyroid hormones are thyroxine (T₄ ), a prohormone product of the thyroid gland, and the more potent 3,5,3-triiodothyronine (T₃ ), a product of both the thyroid and extrathyroidal enzymatic deiodination of T₄ . Under normal circumstances, approximately 85% of T₃ is produced outside the thyroid gland. Production of thyroid secretions is maintained by secretion of thyroid-stimulating hormone (TSH) in the pituitary, which in turn is regulated by secretion of thyrotropin-releasing hormone (TRH) in the hypothalamus. Secretion of TSH and TRH appears to be negatively regulated by T₄ and T₃ . Many believe that all effects of thyroid hormones are mediated by T₃ and that T₄ functions as a prohormone.

Thyroid hormones create their effects through several mechanisms. Binding of T₃ to high-affinity nuclear receptors (TR_α and TR_β ) and subsequent activation of DNA-directed mRNA synthesis may account for the anabolic growth and developmental effects, plus some calorigenic effect, of thyroid hormones. Patients with the syndrome of thyroid hormone resistance have point mutations in TR_β . Thyroid hormone is also responsible for an increased concentration of adrenergic receptors, which may account for many of its cardiovascular effects.

Because T₃ has a greater biologic effect than T₄ does, one would expect the diagnosis of thyroid disorders to be based on levels of T₃ . However, this is not usually the case. The diagnosis of thyroid disease is confirmed by one of several biochemical measurements: levels of free T₄ or total serum concentrations of T₄ and the "free T₄ estimate." This estimate is obtained by multiplying total T₄ (free and bound) by the thyroid-binding ratio (formerly called resin T₃ uptake) ( Table 27-14 ). Free T₄ can be accurately

TABLE 27-14 -- Biochemical measurements of thyroid function that account for variation in production of thyroid-binding globulin

	Examples of Normal Thyroid Status
	FT4 E	=	T4	×	THBR	TSH
Normal	0.19	=	0.6	×	31%	0.2
	(0.12–0.25)		(0.4–0.9)		(25%–35%)	(0.2–0.8)
During use of oral contraceptives	0.19	=	1.3	×	15%	0.3
During use of corticosteroids	0.18	=	0.3	×	60%	0.3
FT4 E is the free T4 (thyroxine) estimate. It is usually obtained by multiplying the total T4 concentration (the free amount and the amount bound to protein) by the thyroid hormone-binding ratio (THBR, formerly called the resin T3 uptake). THBR is a measure of the bound thyroid hormone-binding protein. TSH is the thyroid-stimulating hormone secreted by the pituitary in the negative feedback loop. (TSH increases when FT4 E is low in hypothyroidism.)

TABLE 27-15 -- Factors influencing serum levels of thyroxine-binding globulin

Conditions Increasing Serum Levels	Conditions Decreasing Serum Levels
Use of oral contraceptives	Testosterone
Pregnancy	Use of corticosteroids
Use of estrogen	Severe illness
Infectious hepatitis	Cirrhosis
Chronic active hepatitis	Nephrotic syndrome
Neonatal state	Inherited conditions
Acute intermittent prophyria	?Phenytoin
Inherited conditions

measured by many laboratories. Direct measurement of free T₄ obviates the need to account for changes in binding protein synthesis and affinity caused by other conditions. The T₃ -binding ratio measures the extra quantity of serum protein-binding sites. This measurement is necessary because thyroxine-binding globulin (TBG) is abnormally high during pregnancy, hepatic disease, and estrogen therapy (all of which would elevate the total T₄ level) ( Table 27-15 ). Reliable interpretation of measurements of the total hormone concentration in serum necessitates data on the percentage of bound hormone. The thyroid hormone-binding ratio test provides this information. In this test, ¹³¹ I-labeled T₃ is added to a patient's serum and allowed to reach an equilibrium binding state. A resin is then added that binds the remaining radioactive T₃ . Resin uptake is greater if the patient has fewer TBG-binding sites. In normal patients, resin T₃ uptake (the thyroid hormone-binding ratio) is 25% to 35%. When serum TBG is elevated, the thyroid hormone-binding ratio is diminished (see Table 27-14 ). When serum TBG is diminished, as in nephrotic syndrome, in conditions in which glucocorticoids are increased, or in chronic liver disease, the thyroid hormone-binding ratio is increased.

The free T₄ estimate and the free T₃ estimate are frequently used as measures of a patient's serum T₄ and T₃ hormone concentration. To obtain these estimates, the concentration of total serum T₄ or total serum T₃ is multiplied by the measured thyroid hormone-binding ratio.

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Hyperthyroidism can be diagnosed by measuring levels of TSH after administration of TRH. Although administering TRH normally increases TSH levels in blood, even a small increase in the T₄ or T₃ level in blood abolishes this response. Thus, a subnormal or absent serum TSH response to TRH is a very sensitive indicator of hyperthyroidism.^[242] In one group of disorders involving hyperthyroidism, serum TSH levels are elevated in the presence of elevated levels of free thyroid hormone.

Measurement of the α-subunit of TSH has been helpful in identifying the rare patients who have a pituitary neoplasm and who usually have increased α-subunit concentrations. Some patients are clinically euthyroid in the presence of elevated levels of total T₄ in serum. Certain drugs, notably gallbladder dyes, propranolol, glucocorticoids, and amiodarone, block the conversion of T₄ to T₃ , thereby elevating T₄ levels. Severe illness also slows this conversion. Levels of TSH are often high in situations in which the rate of conversion is decreased. In hyperthyroidism, cardiac function and responses to stress are abnormal; return of normal cardiac function parallels the return of TSH levels to normal.

Hyperthyroidism

Although hyperthyroidism is usually caused by the multinodular diffuse enlargement in Graves' disease (also associated with disorders of the skin or eyes, or both), it can also occur with pregnancy,^[243] thyroiditis (with or without neck pain), thyroid adenoma, choriocarcinoma, or TSH-secreting pituitary adenoma.^[244] Five percent of women have thyrotoxic effects 3 to 6 months postpartum and tend to have recurrences with subsequent pregnancies. Major manifestations of hyperthyroidism are weight loss, diarrhea, warm moist skin, weakness of large muscle groups, menstrual abnormalities, osteopenia, nervousness, jitteriness, intolerance to heat, tachycardia, cardiac arrhythmias, mitral valve prolapse,^[245] and heart failure. When the thyroid is functioning abnormally, the entity most threatened is the cardiovascular system. When diarhea is severe, dehydration should be corrected preoperatively. Mild anemia, thrombocytopenia, increased serum alkaline phosphatase, hypercalcemia, muscle wasting, and bone loss frequently occur in hyperthyroidism. Muscle disease usually involves the proximal muscle groups; it has not been reported to cause respiratory muscle paralysis. In the apathetic form of hyperthyroidism (seen most commonly in persons older than 60 years), cardiac effects dominate the clinical picture.^[246] Signs and symptoms include weight loss, anorexia, and cardiac effects such as tachycardia, irregular heart rhythm, atrial fibrillation (in 10%), heart failure, and occasionally, papillary muscle dysfunction.^{[245] [246]}

Although β-adrenergic receptor blockade can control the heart rate, its use is fraught with hazard in a patient already experiencing CHF. However, a decreasing heart rate may improve heart-pumping function. Thus, hyperthyroid patients who have fast ventricular rates, who are in CHF, and who require emergency surgery are given esmolol guided by changes in pulmonary artery wedge pressure and their condition. If slowing the heart rate with a small dose of esmolol (50 µg/kg) does not aggravate heart failure, we administer more esmolol. We believe that we should aim to avoid imposing surgery on any patient whose thyroid function is clinically abnormal. Therefore, we believe that only "life or death" emergency surgery should preclude making the patient pharmacologically euthyroid, a process that can take 2 to 6 weeks. Antithyroid medications include propylthiouracil or methimazole, both of which decrease the synthesis of T₄ and may enhance remission by reducing TSH receptor antibody levels (the primary pathologic mechanism in Grave's disease). Propylthiouracil also decreases the conversion of T₄ into the more potent T₃ . However, the literature indicates a trend toward preoperative preparation with propranolol and iodides alone.^[247] This approach is quicker (i.e., 7 to 14 days versus 2 to 6 weeks); it shrinks the thyroid gland, as does the more traditional approach; it decreases conversion of the prohormone T₄ into the more potent T₃ ; and it treats symptoms but may not correct abnormalities in left ventricular function.^[248] Regardless of the approach, antithyroid drugs should be administered chronically and on the morning of surgery. If emergency surgery is necessary before the euthyroid state is achieved, if subclinical hyperthyroidism progresses without adequate treatment, ^[249] or if hyperthyroidism gets out of control during surgery, intravenous administration of esmolol, 50 to 500 µg/kg, could be titrated to restore a normal heart rate (assuming the absence of CHF) (see earlier). In addition, intravascular fluid volume and electrolyte balance should be restored. However, administering propranolol or esmolol does not invariably prevent "thyroid storm." ^[250]

No controlled study has demonstrated clinical advantages of any anesthetic drug over another for surgical patients who are hyperthyroid. A review of cases performed at the University of California, San Francisco, from 1968 to 1982 revealed that virtually all anesthetic drugs and techniques^[251] have been used without adverse effects even being remotely attributable to the drug or technique. Furthermore, although some investigators have recommended that anticholinergic drugs (especially atropine) be avoided inasmuch as they interfere with the sweating mechanism and cause tachycardia, atropine has been given as a test for the adequacy of antithyroid treatment. Because patients are now subjected to operative procedures only when euthyroid, the traditional "steal" of a heavily premedicated hyperthyroid patient (so commonly found in the iodine-deficient locales surrounding the Lahey, Mayo, and Cleveland Clinics) in the operating room has vanished.

A patient with a large goiter and an obstructed airway can be handled in the same way as any other patient with problematic airway management. Preoperative medication should avoid excessive sedation, and an airway should be established, often with the patient awake. A firm armored endotracheal tube is preferable and should be passed beyond the point of extrinsic compression. It is most useful to examine CT scans of the neck preoperatively to determine the extent of compression. Maintenance of anesthesia usually presents little difficulty. Postoperatively, extubation should be performed under optimal circumstances

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Of the many possible postoperative complications (nerve injuries, bleeding, and metabolic abnormalities), "thyroid storm" (discussed later), bilateral recurrent nerve trauma, and hypocalcemic tetany are the most feared. Bilateral recurrent laryngeal nerve injury (by trauma or edema) causes stridor and laryngeal obstruction as a result of unopposed adduction of the vocal cords and closure of the glottic aperture. Immediate endotracheal intubation is required, usually followed by tracheostomy to ensure an adequate airway. This rare complication occurred only once in more than 30,000 thyroid operations at the Lahey Clinic. Unilateral recurrent nerve injury often goes unnoticed because of compensatory overadduction of the uninvolved cord. However, we often test vocal cord function before and after this surgery by asking the patient to say "e" or "moon." Unilateral nerve injury is characterized by hoarseness and bilateral nerve injury by aphonia. Selective injury to the adductor fibers of both recurrent laryngeal nerves leaves the abductor muscles relatively unopposed, and pulmonary aspiration is a risk. Selective injury to the abductor fibers leaves the adductor muscles relatively unopposed, and airway obstruction can occur. Bullous glottic edema, an additional cause of postoperative respiratory compromise, has no specific cause or known preventive measure.

The intimate involvement of the parathyroid gland with the thyroid gland can result in inadvertent hypocalcemia during surgery for thyroid disease. Complications relating to hypocalcemia are discussed in the later section "Hypocalcemia."

Because postoperative hematoma can compromise the airway, neck and wound dressings are placed in a crossing fashion (rather than vertically or horizontally) and should be examined for evidence of bleeding before a patient is discharged from the recovery room.

Thyroid Storm

"Thyroid storm" is the name for the clinical diagnosis of a life-threatening illness in a patient whose hyperthyroidism has been severely exacerbated by illness or surgery. Thyroid storm is characterized by hyperpyrexia, tachycardia, and striking alterations in consciousness.^[251] It can thus be manifested very similarly to malignant hyperthermia, pheochromocytoma, or neuroleptic malignant syndrome.^[210] No laboratory tests are diagnostic of thyroid storm, and the precipitating (nonthyroidal) cause is the major determinant of survival. Therapy can include blocking the synthesis of thyroid hormones by administering antithyroid drugs, blocking the release of preformed hormone with iodine, meticulous attention to hydration and supportive therapy, and correcting the precipitating cause. Blocking the sympathetic nervous system with reserpine, guanethidine, or α- and β-receptor antagonists may be exceedingly hazardous and requires skillful management and constant monitoring of the critically ill patient.

Thyroid dysfunction, either hyperthyroidism or hypothyroidism, develops in more than 10% of patients treated with the antiarrhythmic agent amiodarone. ^[252] Approximately 35% of the drug's weight is iodine, and a 200-mg tablet releases about 20 times the optimal daily dose of iodine. This iodine can lead to reduced synthesis of T₄ or increased synthesis. In addition, amiodarone inhibits the conversion of T₄ into the more potent T₃ .

Patients receiving amiodarone might be considered to be in need of special attention preoperatively and may even require special attention to anesthesia, not just because of the arrhythmia that led to such therapy but also to ensure that no perioperative dysfunction or surprises occur because of unsuspected thyroid hyperfunction or hypofunction.^[253] Many patients with amiodarone-induced thyrotoxicosis receive steroids for a period of time, another area of questioning that might be triggered by the use of amiodarone in a preoperative patient.

Hypothyroidism

Hypothyroidism is a common disease that occurs in 5% of a large population in Great Britain, in 3% to 6% of a healthy older population in Massachusetts, and in 4.5% of a medical clinic population in Switzerland. The apathy and lethargy that often accompany hypothyroidism frequently delay its diagnosis, so the perioperative period may be the first opportunity to spot many such hypothyroid patients. However, hypothyroidism is usually subclinical, serum concentrations of thyroid hormones are in the normal range, and only serum TSH levels are elevated.^{[254] [255]} The normal range of TSH is 0.3 to 4.5 mU/L, and TSH values of 5 to 15 mU/L are characteristic of this entity.^[255] In such cases, hypothyroidism may have little or no perioperative significance. However, a retrospective study of 59 mildly hypothyroid patients found that more hypothyroid patients than control subjects required prolonged postoperative intubation (9 of 59 versus 4 of 59) and had significant electrolyte imbalances (3 of 59 versus 1 of 59) and bleeding complications (4 of 59 versus 0 of 59).^[256] Because only a small number of charts were examined, these differences did not reach statistical significance. In another study, overt hypothyroidism later developed in a high percentage of patients with a history of subclinical hypothyroidism.^[257] Thus, a previous history of subclinical hypothyroidism may indicate the need to search for and be concerned about the possibility of overt hypothyroidism.

In the less frequent occurrences of overt hypothyroidism, the relative lack of thyroid hormone results in slow mental functioning, slow movement, dry skin, arthralgias, carpal tunnel syndrome, periorbital edema, intolerance to cold, depression of the ventilatory responses to hypoxia and hypercapnia,^[258] impaired clearance of free water with or without hyponatremia, "hung-up reflexes," slow gastric emptying, sleep apnea,^[259] and bradycardia. In extreme cases, cardiomegaly, heart failure, and pericardial pleural effusions are manifested as fatigue, dyspnea, and orthopnea.^[260] Hypothyroidism is often associated with amyloidosis, which may produce an enlarged tongue, abnormalities of the cardiac conduction system, and renal disease. Hypothyroidism decreases the anesthetic requirement slightly.^[261] The tongue may be enlarged in a hypothyroid patient even in the absence of amyloidosis, and such enlargement may hamper endotracheal intubation.

A rising TSH level is a most sensitive indicator of failing thyroid function. Ideal preoperative and preprocedure

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Treating hypothyroid patients with symptomatic coronary artery disease poses special problems and may require compromises in the general practice of preoperatively restoring euthyroidism with drugs.^[260] Although both levothyroxine and esmolol may be given, adequate amelioration of both ischemic heart disease and hypothyroidism may be difficult to achieve. The need for thyroid therapy must be balanced against the risk of aggravating the anginal symptoms. One review suggested early consideration of coronary artery revascularization. ^[263] It advocated initiating thyroid replacement therapy in the ICU soon after the patient's arrival from the operating room and myocardial revascularization surgery. However, several deaths from arrhythmia and CHF, as well as cardiogenic shock with infarction, have occurred while patients who were not given thyroid therapy were awaiting surgery. Thus, true emergency coronary artery revascularization should be considered in patients with both severe coronary artery disease and significant hypothyroidism.

In hypothyroidism, respiratory control mechanisms do not function normally.^[258] However, the response to hypoxia and hypercapnia and the clearance of free water become normal with thyroid replacement therapy.^{[258] [260]} Drug metabolism is anecdotally reported to be slowed, and awakening times from sedatives are reported to be prolonged during hypothyroidism. However, few formal studies and none in humans of the pharmacokinetics and pharmacodynamics of sedatives or anesthetic drugs have been published.^[264] These concerns disappear when thyroid function is normalized preoperatively. Addison's disease (with its relative steroid deficiency) is more common in hypothyroidism, and some endocrinologists routinely treat noniatrogenic hypothyroid patients with stress doses of steroids perioperatively because both conditions are commonly caused by autoimmune responses. The possibility that this steroid deficiency exists should be considered if the patient becomes hypotensive perioperatively. Body heat mechanisms are inadequate in hypothyroid patients, so temperature should be monitored and maintained, especially in patients requiring emergency surgery. Because of an increased incidence of myasthenia gravis in hypothyroid patients, it may be advisable to use a peripheral nerve stimulator to guide administration of muscle relaxants (see Chapter 39 ).

Thyroid Nodules and Carcinoma

More than 90% of thyroid nodules are benign, yet identifying malignancy in a solitary thyroid nodule is a difficult and important procedure. Male patients and patients with previous radiation therapy to the head and neck have an increased likelihood of malignant disease in their nodules. Often, needle biopsy and scanning are sufficient for the diagnosis, but occasionally, excisional biopsy is needed. Papillary carcinoma accounts for more than 70% of all thyroid carcinomas. Simple excision of lymph node metastases appears to be as efficacious for patient survival as radical neck procedures. Follicular carcinoma accounts for about 15% of thyroid carcinomas, is more aggressive, and has a less favorable prognosis.

Medullary carcinoma, the most aggressive form of thyroid carcinoma, is associated with a familial incidence of pheochromocytoma, as are parathyroid adenomas. For this reason, a history might be obtained from patients who have a surgical scar in the thyroid region so that the possibility of occult pheochromocytoma can be ruled out.