Thyroid hormones T3 and T4 are in a normal range at 3 to 11 and 4.5 to 11.2 micrograms per deciliter of blood, respectively, according to Medline Plus. When hormone levels are above 8 to 11 micrograms, this is considered overactive thyroid, states Progressive Health.
High levels of thyroid hormones can cause hyperthyroidism, resulting in a variety of symptoms, WebMD states. A low level of thyroid hormone is called hypothyroidism, which leads to low energy. The thyroid is a small gland at the front of the throat influencing metabolism, growth and body temperature. Poorly functioning thyroid can cause minor issues such as goiter and or more serious problems such as thyroid cancer.