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C18H24O3, a female sex hormone of the estrogen group with the chemical formula
Estriol occurs as colorless crystals with a melting point of 280°C. It is readily soluble in organic solvents. Optically active, it has a specific rotation of [α]D = +61°. Estriol was first isolated in 1934 from the urine of pregnant women. The hormone is ten to 15 times weaker in its physiological activity than estrone, from which it may be easily obtained semisynthetically.
In humans, dogs, and rats, estriol is one of the end products of the metabolism of estrogen. Estriol has also been discovered in the fruits and flowers of certain plants, including the willow, milkweed, and wheat. Seasonal changes or unusual local conditions sometimes cause an increase in the amount of the hormone contained in forage grasses. As a result, cattle may suffer from certain diseases or may produce less milk, since estriol inhibits lactation. Estriol has important physiological functions (see).