This hormone is produced by the thyroid gland and regulates the metabolic processes of the body. High levels are seen in the disease hyperthyroidism. Symptoms include weight loss, tremor of hands, anxiety and increased sweating. Hypothyroidism (low levels) is due to reduced production by the gland and results in slowing down of metabolism with mental dullness, physical slowness and weight gain. The gland can be enlarged due to cysts or cancer and the level of free T4 may or may not be affected.