Primary malfunction of the thyroid gland may result in excessive (hyper) or below normal (hypo) release of T3 or T4. In addition, as thyroid function is directly affected by TSH, malfunction of the pituitary or the hypothalamus influences the thyroid gland activity. Disease in any portion of the thyroid-pituitary-hypothalamus system may influence the levels of T3 and T4 in the blood. Diagnostically, T3 concentration is more sensitive to certain thyroid conditions than T4. While T4 levels are a sensitive (and superior) indicator of hypothyroidism, T3 blood levels better define hyperthyroidism. Because T3 concentration in serum changes faster and more markedly than T4, the T3 level is also an excellent indicator of the ability of the thyroid to respond to both stimulatory and suppressive tests. Under conditions of strong thyroid stimulation, the T3 level offers a good estimation of thyroidal reserve as well.