An increased incidence of this disorder in monozygotic twins points to an inherited factor, probably an autosomal recessive gene.

This disease occasionally coexists with other endocrine abnormalities, such as diabetes mellitus, thyroiditis, and hyperparathyroidism.

Thyrotoxicosis may also be caused by the production of autoantibodies (thyroid-stimulating immunoglobulin and thyroid-stimulating hormone [TSH]-binding inhibitory immuno-globulin), possibly because of a defect in suppressor-T-lymphocyte function that allows the formation of autoantibodies.

In latent thyrotoxicosis, excessive dietary intake of iodine and, possibly, stress can precipitate clinical thyrotoxicosis.

In a person with inadequately treated thyrotoxicosis, stress—including surgery, infection, toxemia of pregnancy, and diabetic ketoacidosis—can precipitate thyroid storm.

Central nervous system: difficulty in concentrating because increased thyroxine secretion accelerates cerebral function; excitability or nervousness due to increased basal metabolic rate; fine tremor, shaky handwriting, and clumsiness from increased activity in the spinal cord area that controls muscle tone; emotional instability and mood swings, ranging from occasional outbursts to overt psychosis

Skin, hair, and nails: smooth, warm, flushed skin (patient sleeps with minimal covers and little clothing); fine, soft hair; premature graying and increased hair loss in both sexes; friable nails and onycholysis (distal nail separated
from the bed); pretibial myxedema (dermopathy), producing thickened skin; and accentuated hair follicles, raised red patches of skin that are itchy and sometimes painful, with occasional nodule formation. Microscopic examination shows increased mucin deposits.

Cardiovascular system: tachycardia; full, bounding pulse; wide pulse pressure; cardiomegaly; increased cardiac output and blood volume; visible point of maximal impulse; paroxysmal supraventricular tachycardia and atrial fibrillation (especially in elderly people); and, occasionally, a systolic murmur at the left sternal border

Respiratory system: dyspnea on exertion and at rest, possibly from cardiac decompensation and increased cellular oxygen utilization

GI system: excessive oral intake with weight loss; nausea and vomiting due to increased GI motility and peristalsis; increased defecation; soft stools or, with severe disease, diarrhea; and liver enlargement

Musculoskeletal system: weakness (especially in proximal muscles), fatigue, and muscle atrophy; rare coexistence with myasthenia gravis; possibly generalized or localized paralysis associated with hypokalemia; and occasional acropachy (soft-tissue swelling, accompanied
by underlying bone changes where new bone formation occurs)

Reproductive system: in females, oligomenorrhea or amenorrhea, decreased fertility, higher incidence of spontaneous abortions; in males, gynecomastia due to increased estrogen levels; in both sexes, diminished libido

Eyes: exophthalmos (produced by the combined effects of accumulation of mucopolysaccharides and fluids in the retroorbital tissues that force the eyeball outward, and of lid retraction that produces the characteristic staring gaze); occasional inflammation of conjunctivae, corneas, or eye muscles; diplopia; and increased tearing.