Hormones can be classified into steroids, eicosanoids, amino acid-based derivatives, and peptide hormones based on their structures. Steroids and eicosanoids are lipid-soluble hormones. Steroids are derived from cholesterol and contain four connected carbon rings with different side chains attached. Estradiol, released from ovaries, and testosterone from testes are examples of steroid hormones. Eicosanoids are derived from arachidonic acid and include prostaglandins and leukotrienes Amines, peptides, and glycoproteins are water-soluble hormones. Amine hormones are synthesized from a single amino acid. Epinephrine is a tyrosine derivative. It is also known as adrenaline and triggers the fight-or-flight response during stressful situations. Melatonin is a tryptophan derivative that regulates the sleep-wake cycle. Peptide hormones are chains of amino acids. The pancreas produces the peptide hormone insulin after a meal, which stimulates cells to absorb excess glucose from the blood. Some peptide hormones, such as the thyroid-stimulating hormone, are glycoproteins with carbohydrate chains attached to the peptide.