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")