In people with the typical and most common form of hypothyroidism, TSH levels are high and the levels of thyroid hormone T4 are low, states EndocrineWeb. However, in rare cases of hypothyroidism due to pituitary failure, both T4 and TSH levels are low.
TSH, or thyroid-stimulating hormone, is produced by the pituitary gland and stimulates the thyroid gland to produce more thyroid hormone, states EndocrineWeb. In hypothyroidism, the thyroid gland is underactive and does not produce enough thyroid hormone. This stimulates the pituitary gland to produce more TSH to stimulate the thyroid gland to produce more thyroid hormone. This is why in hypothyroidism, TSH levels are high.