T4 levels are typically reported in units such as micrograms per deciliter (µg/dL) or nanomoles per liter (nmol/L). T4 levels are an important indicator of thyroid function, and abnormal T4 levels can indicate hypo- or hyperthyroidism. Low T4 levels may suggest an underactive thyroid gland (hypothyroidism), while high T4 levels may indicate an overactive thyroid gland (hyperthyroidism).