Free Estriol

Clinical information

Estradiol, estrone and estriol belong to the most important endogenous estrogens. They are biosynthesised in the ovaries and testes and the adrenal gland. Starting from cholesterol, androstenedione and testosterone are formed via pregnenolone and are then transformed into estrogens in the gonads and the peripheral tissue. Estrogens are responsible for the development and functioning of the female sex organs. Their finely tuned interaction regulates the menstrual cycle and pregnancy. Hormonal imbalances lead to fertility and menstrual disorders. With menopause, the production of estrogens decreases successively and settles at a low level in postmenopause.

Estradiol is the most effective natural estrogen with a high affinity to the estrogen receptor. It plays a role in the processes of the menstrual cycle and pregnancy. It is present at the highest concentration in the fertile years. During and after menopause, the estradiol concentration drops significantly.

Estrone only has a weak estrogenic effect. While it has only a limited relevance before menopause, it is the estrogen which is present at the highest concentration after menopause. Conjugated estrogens, especially estrone sulphate, serve as a reservoir in the blood stream and can be reactivated by splitting of the sulphate residue in the liver.

High estriol concentrations are mainly detected during pregnancy.


Estriol is determined in blood to exclude pregnancy complications (placental insufficiency) and in prenatal diagnostics (e. g. risk assessment for Down’s syndrome). Increased estriol values are observed in multiple pregnancies, reduced estriol levels may indicate placental insufficiency and diseases or malformation of the foetus.

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