Cardiac: Palpitations and tachycardia are common due to both increased cardiac contractility and increased
Table20.2CausesofThyrotoxicosis
TSH, Thyroid-stimulatinghormone.*AssociatedwithincreasedTSH;allothercausesofthyrotoxicosisassociatedwithdecreasedTSH.
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Fig.20.7Apatientwithhyperthyroidism.Awide-eyed,staringgaze,causedbyoveractivityofthesympatheticnervoussystem,isoneoftheclassicfeaturesofthisdisorder.InGravesdisease,oneofthemostimportantcausesofhyperthyroidism,accumulationoflooseconnectivetissuebehindtheorbitsalsoaddstotheprotuberantappearanceoftheeyes.
peripheral oxygen requirements. Older adult patients with preexisting heart disease may develop congestive heart failure.
Neuromuscular: Patients frequently experience nervousness, tremor, and irritability due to the sympathetic overactivity. Nearly 50% develop proximal muscle weakness (thyroid myopathy).
Ocular changes often call attention to hyperthyroidism. A wide, staring gaze and lid lag are present because of sympathetic overstimulation of the superior tarsal muscle (Müller’s muscle), which functions alongside the levator palpebrae superioris muscle to raise the upper eyelid (
Fig. 20.7 ). However, fullblown thyroid ophthalmopathy associated with proptosis is a feature seen only in Graves disease (discussed later).
Thyroid storm designates the abrupt onset of severe hyperthyroidism. It occurs most often in patients with Graves disease and probably results from an acute elevation in catecholamine levels, as might be encountered during infection, surgery, cessation of anti-thyroid medication, or any form of stress. Thyroid storm is a medical emergency, as a significant number of untreated patients die of cardiac arrhythmias.
Apathetic hyperthyroidism refers to thyrotoxicosis occurring in older adults, in whom the typical features of thyroid hormone excess often are blunted. The underlying thyroid disease is usually detected during laboratory workup for unexplained weight loss or worsening cardiovascular disease.
The diagnosis of hyperthyroidism is based on clinical features and laboratory data. The measurement of serum TSH is the most useful single screening test for hyperthyroidism, because TSH levels are decreased even at the earliest stages, when the disease may still be subclinical. In rare cases of pituitaryor hypothalamus-associated (secondary) hyperthyroidism, TSH levels are either normal or raised. A low TSH value usually is associated with increased levels of free T4. In the occasional patient, hyperthyroidism results predominantly from increased circulating levels of T3 (T3 toxicosis). In such cases, free T4 levels may be decreased, and direct measurement of serum T3 may be useful. Once the diagnosis of thyrotoxicosis has been confirmed by a combination of TSH and free thyroid hormone assays, measurement of radioactive iodine uptake by the thyroid gland can be valuable in determining the etiology. For example, such scans may show diffusely increased uptake in Graves disease, increased uptake in a solitary nodule in toxic adenoma, or decreased uptake in thyroiditis.
Hypothyroidism is caused by structural or functional derangements that interfere with thyroid hormone production. This disorder may be divided into primary and secondary categories, depending on whether it arises from an intrinsic abnormality in the thyroid or from hypothalamic or pituitary disease (
Table 20.3 ). Primary hypothyroidism can be secondary to congenital, autoimmune, or iatrogenic causes.
Genetic defects that perturb thyroid development (thyroid dysgenesis) or the synthesis of thyroid hormone (dyshormonogenetic goiter) are rare overall but the most common cause of congenital hypothyroidism in the U.S.