HYPOTHYROIDISM The thyroid gland produces two related hormones, thyroxine (T4) and triiodothyronine (T3). Hypothyroidism (underactive thyroid) is a condition in which your thyroid gland doesn’t produce enough of certain crucial hormones. Types Primary Hypothyroidism Deficient thyroid hormone secretion can be due to thyroid failure. Secondary Hypothyroidism Deficient thyroid hormone secretion can be due to pituitary