Progesterone
The steroid female sex hormone, synthesized in the yellow body, placenta, small amounts in the seminal vesicles, and the adrenal cortex. The main sources of progesterone in women with a normal menstrual cycle are the yellow body, and during pregnancy – the placenta (from the 6th week of pregnancy it secretes up to 250 mg/24 hours). Progesterone in the blood binds to corticosteroid-binding globulin transcortin (~18%) and albumin (80%). About 2% is free (biologically active). Progesterone is an intermediate product of the biosynthesis of corticosteroids, androgens, and estrogens. In mitochondria, a side chain is cleaved from cholesterol to form pregnenolone. During side chain cleavage, two hydroxylation reactions occur – first at C-22, and then at C-20. Catalyzed by the D4,5 isomerase and 3b-hydroxysteroid dehydrogenase system, progesterone is formed from pregnenolone. Pregnenolone is an intermediate compound in the biosynthesis of all corticosteroids. During the follicular phase of the menstrual cycle, the concentration of progesterone in plasma is usually not higher than 5 nmol/l, while in the luteal phase it increases to 40-50 nmol/l. Two groups of female hormones have been identified, differing in chemical structure and biological functions: estrogens (with estradiol as the main representative) and progestins (with progesterone as the main representative).
When entering target cells, progesterone binds to specific nuclear receptors. The formed hormone-receptor complex interacts with the DNA activator (promoter) and activates gene transcription. The half-life of progesterone in the blood is 5 minutes. In the liver, the hormone conjugates with glucuronic acid and is excreted in the urine. The main function of progesterone is to support the reproductive functions of the body. When progesterone is formed, the basal body temperature increases by 0.2-0.5 °C. This happens immediately after ovulation and continues throughout the luteal phase. At high concentrations of progesterone, it interacts with receptors in the renal tubules, thereby competing with aldosterone. Due to competitive inhibition of aldosterone, it loses the ability to stimulate sodium reabsorption. Progesterone promotes thickening of the uterine lining, facilitates the implantation of the fertilized egg. After fertilization, it reduces the uterus’s response to various substances that induce contractions. By determining the concentration of progesterone in the blood, it is confirmed that ovulation is taking place in the body. By conducting progesterone tests at various times, the exact day of ovulation can be determined. A decrease in progesterone concentration is detected in case of a disrupted luteal phase (short luteal phase syndrome).
If a miscarriage is imminent or the fetus is nonviable, the concentration of progesterone decreases compared to the norm, which must correspond to the respective moment of pregnancy. If an ectopic pregnancy is suspected, determining the concentration of progesterone can confirm the diagnosis.
Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė