In newborns and infants with DS, the most common reason for hypothyroidism is that the thyroid did not form correctly in the fetus. The symptoms of low thyroid hormone are difficult to pick up, especially in infants. In older infants and children, a T4 and TSH is recommended, and some doctors include a measurement of T3 as well. While the cause isn't clear, this may reflect a regulatory defect of TSH, or it may be a sign of impending true hypothyoidism. Some endocrinologists will recommend retesting in 3 to 6 months, and others will recommend treating as if it were an early hypothyroid state.
Treatment is the replacement of thyroid hormone with synthetic thyroxine.
The dose is managed by watching sequential blood tests to see how the thyroid responds. Treatment is usually needed for life. While not anywhere near as common as hypothyroidism, this condition does occur. The usual cause is, again, autoimmune disease, but in hyperthyroidism, the immune system cranks up the thyroid. Symptoms include rapid heart rate, nervousness, sweating, decreased attention span, flushed skin, always feeling hot and loss of hair.