T3, T4 and TSH are the most important indicators to determine thyroid function. TSH is a thyroid stimulating hormone secreted by the pituitary gland, while T3 and T4 are hormones secreted by the thyroid gland. there are two main types of conditions in which TSH is reduced and T3 and T4 are normal: i. Pituitary hypothyroidism, in which the pituitary gland secretes reduced TSH and T3 and T4 are normal, is also the most common type of secondary hypothyroidism. Second, subclinical hyperthyroidism, where the pituitary gland secretes normal TSH and the thyroid gland secretes increased T3 and T4, and the feedback suppresses TSH, but the sensitivity of TSH is higher than that of T3 and T4, so there is a situation of reduced TSH and normal T3 and T4. Which is the specific case will be determined together with other tests.