The thyroid gland is responsible for producing two hormones: T3 and T4. Their secretion is controlled by TSH, a hormone produced by a small gland known as the pituitary gland (hypophysis) located at the base of the brain. When T3 and T4 levels are too low to meet the body’s energy needs, the pituitary gland secretes more TSH to stimulate the thyroid gland. Therefore, when T3 and T4 levels are low, it is normal for TSH levels to be high. When the TSH levels are also low, it means there could be a problem with the pituitary gland (secondary hypothyroidism) and further investigation is necessary.
Sick euthyroid syndrome shows up in reduced levels of T3, T4 and TSH in individuals who have normal thyroid and pituitary gland function but who are suffering from a major health condition (anorexia, trauma, hemorrhagic shock, severe infection, etc.). In these situations, the body produces reverse T3 (rT3), which is an inactive form of T3. In rare cases where other thyroid function tests do not confirm the diagnosis, high levels of reverse T3 help rule out a diagnosis of secondary hypothyroidism.