How are steroids structured at the molecular level?

All steroids are characterized by a core molecular structure known as the steroid nucleus, or gonane. This parent structure consists of 17 carbon atoms arranged into four fused rings, conventionally labeled A, B, C, and D, which are bonded to 28 hydrogen atoms.1

This nucleus is a three-dimensional structure where atoms or groups are attached via spatially directed bonds. The stereochemistry—the specific spatial arrangement—of these bonds is critical to the molecule's identity. For instance, the junction between rings A and B can exist in cis or trans configurations, which significantly alters the molecule's shape.1

Steroids are highly diverse because this core can be modified through the addition, removal, or replacement of atoms. Substituent groups attached to the ring system are designated as $\alpha$ (below the plane) or $\beta$ (above the plane). Additionally, different classes of steroids are distinguished by variations in the size and structure of the side chain attached at position 17 of the carbon skeleton.1