Lesson 8: Accompanying Conditions and Complications
Bone Loss and Muscle Weakness
Hyperthyroidism, especially long-term untreated conditions of both overt and subclinical hyperthyroidism can eventually cause bone loss and muscle weakness.
Bone Loss
Thyroid hormone has a direct effect on bone resorption. Bone resorption is the process in which bone cells are broken down. In hyperthyroidism bone resorption is increased to a higher degree than bone formation. Consequently, bone loss can develop over time and patients experience an increased fracture rate.
Patients with nodular goiter and conditions of subclinical thyrotoxicosis were shown in one study to have lower bone density than subjects in their age group with normal thyroid function. Two years after treatment to reduce thyroid hormone levels, postmenopausal women in this study had higher bone density than subjects in the study group who did not receive treatment. Although studies are controversial, hypothyroid patients receiving high doses of thyroid replacement hormone do not have a lower bone density than comparable subjects.
Muscle Weakness
Muscle weakness and wasting are common clinical manifestations of thyrotoxicosis. All patients with clinically evident thyrotoxicosis are reported to have some degree of muscle weakness although it is often not the predominant feature in younger patients. The extent and severity of muscle weakness or myopathy tends to be related to the duration of thyrotoxicosis but not its severity. Muscle weakness is generally more pronounced in men than women. Proximal muscle weakness, that is, weakness of the large muscles closest to the inside of the body such as thigh muscles, is usually more pronounced than distal muscle weakness. Muscle pain is rare compared to muscle weakness.
Because the proximal muscles of the shoulders and thighs are most likely to be affected patients experience difficulty climbing stairs, rising from a seated position, or performing tasks that require the arms to be raised above the head. Other muscle groups are rarely involved, although occasional patients may complain of weakness in the pharyngeal, facial or laryngeal muscles, which causes difficulty swallow, facial muscle weakness or voice changes.
The hands may also be weak although hand tremor is a more common symptoms. Muscular atrophy or shrinkage may be present on physical examination, usually in the proximal muscles although the pectoral muscles and muscles of the pelvic girdle may also be affected. In older patients, up to 50 percent of patients may be affected. Because most patients receive a prompt diagnosis and immediate treatment, muscle atrophy is rarely seen today.
Serum creatinine level are normal or low in thyrotoxicosis even in patients with severe muscle weakness. Creatinine is a muscle enzyme that’s increased in muscle damage. Creatinine levels are high in overt hypothyroidism. Muscle biopsy is usually normal in thyrotoxicosis although there may be varying degrees of fiber atrophy, fatty infiltration and nerve fiber damage caused by the effects of excess thyroid hormone on muscle. Studies show that muscle weakness in thyrotoxicosis is related to low levels of vitamin B6.
Periodic Paralysis
Rarely, patients with thyrotoxicosis have periodic episodes of paralysis identical to those that occur in patients with familial hypokalemic periodic paralysis, a condition of low potassium. Most hyperthyroid patients with periodic paralysis are Asian men although it has also been seen in women and people of all races. The lower extremities are affected more than the upper extremities although weakness may be more generalized and result in impaired breathing. Onset of symptoms is sudden but may be preceded by muscle stiffness or cramps.
The weakness generally lasts from a few hours to several days, and some muscle weakness and soreness may persist longer. These episodes are more common in warm weather and at night. They’re also known to occur after strenuous activity followed by rest, meals with high carbohydrate content, alcohol ingestion, trauma and emotional stress. The muscles most strenuously affected are the most likely to be severely affected.
These episodes only occur when the patient is thyrotoxic although the thyrotoxicosis may be mild with the periodic paralysis being the only symptom. The major biochemical change is the low potassium level, which is related to a shift of potassium from the plasma into the blood cells.
