Thyroid Storm

Chapter 72 Thyroid Storm

Cynthia R. Ward, VMD, PhD, DACVIM

• Thyroid storm is a newly described syndrome of acute thyrotoxicosis in veterinary patients that occurs primarily in hyperthyroid cats.

• The pathogenesis is unknown but probably results from rapid increases in serum thyroid hormone levels coupled with activation of the sympathetic nervous system.

• An event often triggers thyroid storm, although the event may not be readily apparent.

• Clinical signs may include hyperthermia, central nervous system disturbances, acute or severe vomiting and diarrhea, abdominal pain, icterus, cardiac murmurs with or without arrhythmias, pleural effusion, pulmonary edema, tachypnea, hypertension, retinopathies, extreme muscle weakness and cervical ventroflexion, thromboembolic disease, and sudden death.

• Diagnosis is based on clinical signs and evidence of elevated serum thyroid hormones.

• Treatment is aimed at reducing production and secretion of thyroid hormones, blocking peripheral actions of thyroid hormones, systemic support, and elimination of the precipitating event.

• Successful outcome depends on rapid recognition of the clinical syndrome and aggressive therapy.

INTRODUCTION

Thyroid storm is a syndrome described in human medicine to define a multisystem disorder resulting from organ exposure to excessive amounts of thyroid hormone. This form of acute thyrotoxicosis can be life threatening and is a significant cause of mortality in human emergency rooms. Thyrotoxicosis describes any condition in which there is an excessive amount of circulating thyroid hormone, whether from excess production and secretion from an overactive thyroid gland, leakage from a damaged thyroid gland, or from an exogenous source. In contrast, hyperthyroidism describes thyroid gland hyperfunction. Therefore acute thyrotoxicosis may occur in the absence of thyroid gland hyperfunction, although this is rare in veterinary medicine.

In humans, thyroid storm can occur at any age. It can be present in euthyroid patients as well as treated and partially treated hyperthyroid patients. Although this is a well-recognized syndrome in human medicine, it has not been described as a clinical entity in veterinary medicine. Hyperthyroidism is common in older feline patients. It is also seen rarely in dogs with thyroid carcinoma or extreme oversupplementation of thyroid replacement hormone in hypothyroid dogs. Hyperthyroid cats that experience an acute exacerbation of thyrotoxicosis may be said to have thyroid storm. This chapter will discuss the human syndrome and define a similar syndrome in hyperthyroid veterinary patients. The clinical signs of and treatment modalities for feline patients suffering from thyroid storm will also be presented.

PATHOGENESIS

Although the exact pathogenesis of thyroid storm remains to be elucidated, several factors appear to be involved, including high levels of circulating thyroid hormones, rapid increases in circulating thyroid hormones, hyperactivity of the sympathetic nervous system, and an increased cellular response to thyroid hormone.

High Levels of Circulating Thyroid Hormones

Although one would expect circulating thyroid hormones to be increased in patients with thyroid storm, in human medicine there is no difference between serum thyroid hormone levels in these patients and in more stable hyperthyroid patients. The diagnosis is therefore made primarily based on clinical signs.¹

Rapid, Acute Increases in Circulating Thyroid Hormones

The magnitude of change in the serum thyroid hormone levels may be more important than the actual levels themselves. This would explain the occurrence of thyroid storm following radioactive iodine therapy and thyroid surgery that damage the thyroid gland and cause release of hormone or following abrupt cessation of antithyroid medication resulting in the rapid rise of serum thyroid hormone levels.² Certainly, nonthyroidal illness, known to be a precipitating factor in human medicine, has been shown to alter binding of thyroid hormones to their carriers and could be responsible for rapid alteration of circulating thyroid hormone as well.

Hyperactivity of the Sympathetic Nervous System

Activation of the sympathetic nervous system has been implicated in the onset of thyroid storm.³ Many of the clinical signs and physiologic signs are similar to those seen during catecholamine excess. Serum and urine catecholamine levels in humans are within normal limits during thyroid storm. However, thyroid hormones can alter tissue sensitivity to catecholamines at the cell surface receptor as well as the intracellular signaling levels, and this increased sensitivity may result in the clinical signs seen during thyroid storm. In addition, many of these clinical signs are controlled by β-adrenergic blockade.

Increased Cellular Response to Thyroid Hormones

This effect has been implicated in the cause of thyroid storm resulting from infection, sepsis, hypoxemia, hypovolemia, and lactic acidosis or ketoacidosis.⁴

PRECIPITATING EVENTS

In most cases of thyroid storm in humans, a precipitating event can be identified, although no known cause is found in up to 2% of cases.⁵ The most common precipitating events are infection, thyroidal and nonthyroidal surgery, radioactive iodine therapy, administration of iodinated contrast dyes, administration of stable iodine, withdrawal of antithyroid medication, amiodarone therapy, ingestion of excessive amounts of exogenous thyroid hormone, vigorous palpation of the thyroid gland, severe emotional stress, and a variety of acute nonthyroidal illnesses. Common events that may precipitate thyroid storm in feline hyperthyroid patients may include radioactive iodine therapy, abrupt withdrawal of antithyroid medication, thyroid surgery, vigorous thyroid palpation, stress, administration of stable iodine compounds, as well as any of the other precipitating factors found in humans (Box 72-1).

CLINICAL SIGNS

Thyroid storm is the acute exacerbation of clinical signs of thyrotoxicosis; however, the diagnosis of thyroid storm in human medicine is based on the prevalence of four major clinical signs. These include (1) fever, (2) central nervous system (CNS) effects from mild agitation to seizures or coma, (3) gastrointestinal (GI) and hepatic dysfunction ranging from vomiting, diarrhea, and abdominal pain to unexplained jaundice, and (4) cardiovascular effects including sinus tachycardia, atrial fibrillation, ventricular tachycardia and congestive heart failure. The combination of these clinical signs along with identification of a precipitating event allows for the diagnosis of thyroid storm.⁵ In cats with presumed thyroid storm, many of these clinical signs also occur (Box 72-2).

Tags: Small Animal Critical Care Medicine

Sep 10, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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