TRH and Klinefelter syndrome: Pituitary hyperplasia refers to non-neoplastic growth or enlargement of pituitary gland cells without qualitative changes in cell biology, which include physiological, pathological, syndrome-related (such as Addison disease, Klinefelter syndrome, and Turner syndrome) and idiopathic subtypes.[9] Pituitary hyperplasia caused by primary hypothyroidism is due to the loss of thyroxine feedback inhibition to the hypothalamus, which can induce the overproduction of TRH and hyperplasia of lactotrophs.