Hyperthyroiism is the term for overactive tissue within the thyroid gland, resulting in overproduction and thus an excess of circulating free thyroid hormones: thyroxine (T4), triiodothyronine (T3), or both. The term is also often used more loosely to describe any syndrome of excess thyroid hormone (more properly termed hyperthyroxinemia), regardless of the source. Thyrotoxicosis is the term for symptomatic hyperthyroxinemia. Thyroid hormone is important at a cellular level, affecting nearly every type of tissue in the body. It functions as a stimulus to metabolism, and is critical to normal function of the cell. In excess it overstimulates, causing "speeding up" of various body systems, and thus symptoms: Fast heart beat results in palpitations, a fast nervous system in tremor and anxiety symptoms, a fast digestive system in weight loss and diarrhea. Lack of functioning thyroid tissue results in a symptomatic lack of thyroid hormone, termed hypothyroidism.