Action of the methylxanthines on the pituitary and pituitary-dependent hormones.

High doses of caffeine produce a stresslike neuroendocrine response in rats. This response is characterized by increased serum corticosterone, beta-endorphin, and decreased serum growth hormone and TSH. There is no effect on prolactin. Curiously, another common stimulant, amphetamine, produces a similar endocrine response in rats. Like caffeine, amphetamine decreases serum TSH [36], increases serum corticosterone [37], and has little effect on serum prolactin [38]. Perhaps amphetamine and caffeine as stimulants activate the same neurotransmitter systems and hence have similar endocrine effects. In man, typically consumed oral doses of caffeine have little or no endocrine effects. A person must consume 500 mg caffeine (equivalent to 5 cups of coffee) in one sitting for there to be any possible endocrine effects. Under these circumstances the endocrine changes that might occur would be limited to slight elevations in plasma ACTH, beta-endorphin, and cortisol.