Which substance is secreted by the pituitary gland that stimulates the ovaries or testes to produce their secretions?

The pituitary gland is the master gland that controls the secretion of other glands in our body. 

It secretes hormones to induce the production of sex hormones by the gonads i.e., testes and ovaries. These hormones secreted by the pituitary are called gonad-stimulating hormones. These lead to the secretion of hormones estrogen and testosterone by the ovaries and testis respectively.