Hyperthyroidism is the term for overactive tissue within the thyroid gland,resulting in overproduction and thus an excess of circulating free thyroidhormones: thyroxine(T4), triiodothyronine (T3), or both. Thyroidhormone is important at a cellular level, affecting nearly every type of tissuein the body.
Thyroid hormone functions as a stimulus to metabolism and is critical tonormal function of the cell. In excess, it both over stimulates metabolism andexacerbates the effect of the sympathetic nervous system, causing"speeding up" of various body systems and symptoms resembling anoverdose of epinephrine (adrenaline). These include fast heart beatand symptoms of palpitations, nervous system tremor and anxiety symptoms,digestive system hypermotility (diarrhea), and weight loss.
On the other hand, a lackof functioning thyroid tissue results in a symptomatic lack of thyroid hormone,termed hypothyroidism.
Signs and symptoms
Major clinical signs include weight loss(often accompanied by an increased appetite), anxiety,intolerance to heat,hair loss, muscle aches, weakness, fatigue, hyperactivity, irritability, apathy, depression, polyuria, polydipsia,delirium, tremor, pretibial myxedema, and sweating. Additionally,patients may present with a variety of symptoms such as palpitationsand arrhythmias(notably atrial fibrillation), shortness of breath (dyspnea), loss oflibido, nausea, vomiting, and diarrhea. Longterm untreated hyperthyroidism can lead to osteoporosis.In the elderly, these classical symptoms may not be present.
Neurological manifestations can include tremors, chorea,myopathy,and in some susceptible individuals (particularly of Asian descent) periodic paralysis. An association betweenthyroid disease and myasthenia gravis has been recognized. Thethyroid disease, in this condition, is autoimmune in nature and approximately5% of patients with myasthenia gravis also have hyperthyroidism. Myastheniagravis rarely improves after thyroid treatment and the relationship between thetwo entities is not well understood. Some very rare neurological manifestationsthat are dubiously associated with thyrotoxicosis are pseudotumor cerebri, amyotrophic lateral sclerosis and a Guillain-Barr�-like syndrome.
Minor ocular (eye) signs, which may be present in any type ofhyperthyroidism, are eyelid retraction ("stare"), extra-ocular muscleweakness, and lid-lag. In hyperthyroid stare(Dalrymplesign) the eyelids are retracted upward more than normal (the normalposition is at the superior corneoscleral limbus, where the "white"of the eye begins at the upper border of the iris). Extra-ocular muscleweakness may present with double vision. In lid-lag (von Graefe's sign), when the patient tracks anobject downward with their eyes, the eyelid fails to follow the downward movingiris, and the same type of upper globe exposure which is seen with lidretraction occurs, temporarily. These signs disappear with treatment of thehyperthyroidism.
Neither of these ocular signs should be confused with exophthalmos(protrusion of the eyeball) which occurs specifically and uniquely inhyperthyroidism caused by Graves'disease (Note that not all exophthalmos is caused by Graves' disease, butwhen present with hyperthyroidism is diagnostic of Graves’s disease). Thisforward protrusion of the eyes is due to immune mediated inflammation in theretro-orbital (eye socket) fat. Exophthalmos, when present, may exacerbatehyperthyroid lid-lag and stare.
Thyrotoxic crisis (or thyroid storm)is a rare but severe complication of hyperthyroidism, which may occur when aThyrotoxic patient becomes very sick or physically stressed. Its symptoms caninclude: an increase in body temperature to over 40 degrees Celsius (104degrees Fahrenheit), tachycardia, arrhythmia, vomiting, diarrhea, dehydration,coma and death.