TG and hypothyroidism: Low thyroid hormone levels (hypothyroidism) are known to affect cognition in the general population, and a prospective analysis of 230 patients with a range of primary brain tumors, including meningioma, LGG, HGG, pituitary adenoma, and acoustic neuroma, found low levels of the thyroid hormone tri-iodothyronine to be common (74%) and associated with lower Mini-Mental State Examination (MMSE) scores (77).