Hormones are chemical messengers secreted by endocrine glands into the bloodstream and transported to target cells. There are three main types of hormones: steroids, proteins, and tyrosine derivatives. Steroid hormones enter cells and interact directly with genes, while protein hormones bind to cell membrane receptors and trigger the release of intracellular secondary messengers. The hypothalamus controls the pituitary gland via the portal vein connecting it to the anterior pituitary, and via neurosecretory cells connecting it to the posterior pituitary, which stores hormones like ADH. ADH secretion is regulated by negative feedback in response to osmoreceptors in the hypothalamus.