Testosterone: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Testosterone: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Testosterone is the main androgen, a steroid hormone made from cholesterol. The Leydig cells of the testis form it; in the cell it binds the androgen receptor, which co-determines which genes are read.

The pathway step by step

1. Cholesterol → Pregnenolone CYP11A1 · StAR transport The protein StAR channels cholesterol into the mitochondrion. There, the enzyme CYP11A1 cleaves off part of the side chain, forming pregnenolone, the shared precursor of all steroid hormones. Source 1
2. Pregnenolone → DHEA CYP17A1 CYP17A1 works in two steps: first it attaches an OH group, then it splits off two carbon atoms. This forms DHEA. Source 1
3. DHEA → Androstenedione 3-beta-HSD · NAD+ The enzyme 3-beta-HSD converts the OH group into a keto group and shifts a double bond in the ring system. Source 1
4. Androstenedione → Testosterone 17-beta-HSD3 · NADPH 17-beta-HSD3 turns the keto group at position 17 into an OH group. Testosterone is then complete and passes from the cell into the blood. Source 1, 2
5. Testosterone in blood → Breakdown in the liver Liver enzymes The liver converts testosterone into androsterone and etiocholanolone and couples these to glucuronic acid. In this water-soluble form, it leaves the body via the urine. Source 1
6. Testosterone in blood → Free testosterone · Release from SHBG SHBG holds on to testosterone and releases it only slowly. It thus acts as a buffer between production in the testis and uptake into the cell. Source 3
7. Free testosterone → DHT 5-alpha reductase · NADPH 5-alpha reductase converts testosterone into dihydrotestosterone. DHT binds more tightly to the androgen receptor and keeps the signal going there for longer. Source 1
8. Free testosterone → Estradiol Aromatase (CYP19A1) Aromatase converts a small part of testosterone into estradiol. It is found in adipose tissue, in the brain and in the testis itself, among other places. Source 1
9. Free testosterone → Androgen receptor Inside the cell, testosterone and DHT bind to the same receptor. The loaded receptor then moves into the cell nucleus. Source 1
10. Androgen receptor → Reading genes In the nucleus, the receptor attaches to specific sections of DNA and there regulates which proteins the cell makes. Source 1

Cofactors in this pathway

• NADPH — Electron donor for CYP11A1, CYP17A1, 17-beta-HSD3 and 5-alpha-reductase Source 1
• Iron — Central atom of the haem in the cytochrome P450 enzymes of this route Source 1In the ORY catalogue as a laboratory value: Eisen
• NAD⁺ — Accepts hydrogen at 3-beta-HSD Source 1In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Zinc — Keeps the two zinc fingers in the DNA-binding part of the androgen receptor in shape Source 6In the ORY catalogue as a laboratory value: Zink

What acts on this pathway

• Finasteride — Finasteride inhibits type II 5-alpha reductase. As a result, less testosterone is converted to DHT; this mechanism is described in pharmacology. Source 4
• Anastrozole — Anastrozole blocks aromatase. The conversion of testosterone to estradiol decreases as a result; this mechanism is also described in pharmacology. Source 5

Sources

1. Miller WL, Auchus RJ. The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr Rev 2011 · PubMed 21051590
2. Zirkin BR, Papadopoulos V. Leydig cells: formation, function, and regulation. Biol Reprod 2018 · PubMed 29566165
3. Hammond GL. Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action. J Endocrinol 2016 · PubMed 27113851
4. Yim E, Nole KL, Tosti A. 5alpha-Reductase inhibitors in androgenetic alopecia. Curr Opin Endocrinol Diabetes Obes 2014 · PubMed 25268732
5. Kelly CM, Buzdar AU. Anastrozole. Expert Opin Drug Saf 2010 · PubMed 20923259
6. Davey RA, Grossmann M. Androgen Receptor Structure, Function and Biology: From Bench to Bedside. Clin Biochem Rev 2016 · PubMed 27057074

Related pathways

• Estradiol — cholesterol
• DHEA — cholesterol
• Cholesterol and lipoproteins — cholesterol
• Cortisol — cholesterol
• Progesterone — cholesterol

As of 2026-09-16. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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