The hypothalamus was a region of the brain that secreted hormones. In Humans, it was located above the pituitary gland.
After making his warp 10 flight in 2372, The Doctor detected elevated serotonin levels in Tom Paris' hypothalamus. (VOY: "Threshold")
The Akritirian clamp stimulated acetylcholine production in the hypothalamus. (VOY: "The Chute")
The Vori propaganda techniques used to forcibly train soldiers also targeted the hypothalamus. (VOY: "Nemesis")
In a deleted scene from ENT: "Minefield", in the Denobulan language, the phrase for "the hypothalamus" is "Ya-sah degata". Phlox mentioned this while testing Hoshi Sato's Denobulan while she was recovering after being injured by a Romulan mine in 2152.