How testosterone works in the body: the androgen pathway

Testosterone as an androgen

Testosterone is the principal androgen in the human body. "Androgen" is the name for a family of steroid hormones that share the ability to act on the androgen receptor, and testosterone is the most abundant and widely studied member of that family. Like other steroid hormones, it is built from cholesterol through a series of enzymatic steps, and it functions as a chemical messenger — a molecule that carries instructions from the cells that make it to the cells that respond to it.

In people assigned male at birth, most testosterone is produced by the Leydig cells of the testes, with smaller amounts made by the adrenal glands. In people assigned female at birth, the ovaries and adrenal glands produce testosterone in smaller quantities, where it also serves as a precursor for other hormones. Once released, testosterone travels through the bloodstream, where much of it is bound to carrier proteins such as sex hormone-binding globulin (SHBG) and albumin, while a smaller "free" fraction is available to interact with tissues.

In the United States, testosterone is a prescription-only medication and a controlled substance. This article is educational only — a description of how the androgen pathway is understood to work, not medical advice, a diagnosis, or a recommendation for any person.

The androgen receptor and how the signal is read

Because testosterone is a lipid-soluble steroid, it can pass through the outer membrane of a cell rather than docking on the surface. Inside responsive cells it encounters the androgen receptor (AR), a protein that acts as the pathway's on-switch. When testosterone binds the androgen receptor, the receptor changes shape and the hormone-receptor complex moves into the cell's nucleus.

In the nucleus, the activated receptor attaches to specific DNA sequences known as androgen response elements. From there it is understood to influence which genes are transcribed — effectively adjusting the set of proteins a cell makes. This is called a genomic mechanism because it works through gene expression, and it is the classical way steroid hormones are described as acting. Researchers also describe faster, non-genomic signaling that does not depend on gene transcription, though the genomic route is the most established.

A tissue responds to testosterone largely in proportion to how much androgen receptor it expresses. Cells rich in the receptor read the signal strongly; cells with little receptor read it weakly or not at all. This is why a single circulating hormone is associated with different activity in different parts of the body — the receptor, not just the hormone, determines where the message lands.

How the body converts and modifies testosterone

Testosterone does not always act in its original form. In certain tissues the enzyme 5-alpha-reductase converts it into dihydrotestosterone (DHT), a related androgen that binds the androgen receptor more tightly. DHT is understood to be the more active androgen in tissues such as skin, hair follicles, and the prostate, which is why the same enzyme is discussed in the context of hair and skin biology.

A second enzyme, aromatase, converts a portion of testosterone into estradiol, a form of estrogen. Estradiol acts on a different set of receptors — the estrogen receptors — so this conversion means testosterone also serves as a precursor that feeds the estrogen pathway. The balance between these conversions is part of why the androgen system is described as interconnected with other hormone systems rather than acting in isolation.

Availability is regulated as well. Because most circulating testosterone is bound to SHBG and albumin, the amount of carrier protein present influences how much free hormone is on hand to interact with receptors at any moment. Taken together, direct receptor binding, enzymatic conversion to DHT or estradiol, and protein binding describe the main ways the body shapes how the androgen signal is delivered.

Tissues the androgen pathway is understood to act on

Skeletal muscle expresses the androgen receptor, and androgen signaling is understood to be involved in the maintenance of muscle tissue and in the processes governing muscle protein turnover. Bone is another androgen-responsive tissue; here the pathway is understood to contribute to bone maintenance both through direct receptor activity and, indirectly, through the estradiol produced by aromatization.

Skin and hair follicles respond largely to DHT, which is why the 5-alpha-reductase step matters in those tissues and features in discussions of body and scalp hair. Reproductive tissues rely on androgen signaling during development and for ongoing function, and the pathway is also involved in the production of sperm within the testes alongside other hormones. The bone marrow is androgen-responsive as well, where signaling is understood to participate in the stimulation of red blood cell production.

The central nervous system contains androgen receptors in several regions, and the pathway is described in connection with signaling related to mood, motivation, and libido. Across all of these tissues, the common thread is the receptor: the androgen pathway is understood to act wherever the androgen receptor is expressed, and the nature of the response depends on the specific cell type. Describing these mechanisms is not the same as predicting an outcome for any individual, which depends on many factors a clinician weighs.

How the body regulates testosterone: the HPG axis

Testosterone production is governed by a feedback loop often called the hypothalamic-pituitary-gonadal (HPG) axis. It begins in the hypothalamus, a region of the brain that releases gonadotropin-releasing hormone (GnRH) in pulses. GnRH signals the pituitary gland to release two other hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

LH travels to the testes, where it prompts the Leydig cells to produce testosterone, while FSH is involved in supporting sperm production. As testosterone — and the estradiol derived from it — rises in the bloodstream, those hormones signal back to the hypothalamus and pituitary to slow the release of GnRH and LH. This negative feedback is what keeps the system self-correcting, easing production when levels are ample and allowing it to rise when they fall.

This closed loop is why the androgen pathway is often described as a thermostat rather than a simple faucet. It also explains why several prescription medications discussed in this area are understood to act at different points along the axis — some at the level of the brain and pituitary, others at the level of the gonads. Whether any such medication is appropriate for a particular person is a clinical judgment that belongs to an independent licensed provider, informed by examination and laboratory evaluation that no article can replace.

How prescription review works on OpenDoseRx

On OpenDoseRx, medications related to the androgen pathway are prescription-only, and the process is built so that a clinician — not the shopper — makes the medical decision. You begin by choosing a product and strength, then complete a medical intake covering your health history and other relevant information.

That intake is routed to an independent, licensed U.S. provider who reviews it and decides whether a prescription is appropriate for you. If the provider determines it is, the prescription is sent to a licensed U.S. pharmacy to be filled and shipped to you. If the request is declined, you are not charged for the medication and you receive a full refund. Nothing here replaces a conversation with your own provider, and no product is dispensed without that independent clinical review.