Cortisol crystals, polarised light micrograph. Cortisol is a steroid hormone produced by the adrenal glands, which sit on top of the kidneys. It is released in response to stress, and increases blood pressure and blood sugar levels. Cortisol also suppresses the immune system, so is used in medicine (known as hydrocortisone) as an anti-inflammatory and to treat allergies. Magnification: x45 at 4x5 inch.