Once TSH makes its way to your thyroid, levels are largely dictated by the amount of T3 and T4 in your blood. When T3 and T4 levels are low, the body produces more TSH to stimulate the thyroid. But when T3 and T4 levels are high, the body produces less. Several other factors can also influence TSH levels. Inflammation of the thyroid gland. Genetics. Deficiency or excess of iodine in the diet. Poisonous substances and radiation exposure. Pregnancy. Certain medications. Antidepressants, cholesterol lowering drugs, chemotherapy drugs, steroids. Thyroid cancer. So try to deal with this if you want your TSH to reduce. Common causes of hypothyroidism include an autoimmune disease. Radiation treatment, or surgical removal of the thyroid gland. Replacing thyroid hormone and altering your diet are crucial for the safe and effective treatment of an underactive thyroid.