Hyperthyroid myopathy occurs when the thyroid gland produces too much of the hormone thyroxine, leading to muscle weakness, some muscle wasting in hips and shoulders, and, sometimes, problems with eye muscles.
In the United States, newborn infants between 24 and 72 hours old are tested for congenital thyroid deficiency (cretinism) using a test that measures the levels of thyroxine in the infant's blood.
Hypothyroidism, or underactive thyroid, develops when the thyroid gland fails to produce or secrete as much thyroxine (T4) and triiodothyonine (T3) as the body needs.
Thyroxine (T4)-The thyroid hormone that regulates many essential body processes.
Triiodothyronine (T3)-A thyroid hormone similar to thyroxine but more powerful.