Subcutaneous tissue is the deepest layer of your skin, sitting just below the two layers you can see and touch (the epidermis on the surface and the dermis beneath it). Also called the hypodermis, it’s made mostly of fat and connective tissue. It acts as your body’s insulation, shock absorber, and energy reserve all at once. In healthy individuals, roughly 80% of total body fat is stored here.
Where It Sits and What It’s Made Of
Your skin has three distinct layers. The epidermis is the thin, outermost barrier. The dermis sits beneath it and contains nerve endings, sweat glands, and blood vessels. Below both of those is the subcutaneous tissue, which connects your skin to the muscle and bone underneath.
Three main cell types make up this layer. Fat cells, called adipocytes, are the most abundant. They cluster together into fatty tissue that stores energy your body can draw on later. Fibroblasts produce collagen, the protein that gives connective tissue its structure and flexibility. And immune cells called macrophages patrol the area, attacking bacteria and other harmful invaders that make it past the upper layers of skin.
How Thick It Is Across Your Body
Subcutaneous tissue is not uniform. Its thickness varies dramatically depending on the body site, along with your age, sex, and overall body composition. The thinnest areas tend to be around the neck, where the fat layer averages just under 4 millimeters. The thickest areas are the buttocks, where measurements average around 20 millimeters and can range from about 6 to 38 millimeters. Other areas like the abdomen, thighs, and upper arms fall somewhere in between.
This variation matters for everyday things like how warm different parts of your body feel, how easily you bruise, and where medical injections are given.
What Subcutaneous Tissue Does
This layer serves several functions that are easy to take for granted. Its fat cells store energy in the form of lipids, which your body breaks down when it needs fuel between meals or during exercise. That same fat acts as thermal insulation, helping you retain body heat in cold environments. And because fat is soft and compressible, the subcutaneous layer works as a cushion that protects your muscles, bones, and internal organs from impacts and pressure.
The connective tissue within the hypodermis also anchors your skin to the structures beneath it while still allowing it to slide and move freely. Without this layer, your skin would either float loosely or be rigidly fixed to your muscles.
Its Role as a Hormone-Producing Organ
Subcutaneous fat does more than just store energy. It actively releases hormones and signaling molecules known as adipokines. Some of these, like leptin, help regulate appetite and metabolism by signaling to your brain how much energy you have in reserve. Others influence inflammation throughout the body. Adiponectin, for example, has anti-inflammatory effects, while molecules like resistin and chemerin promote inflammation. The balance between these signals plays a role in metabolic health, heart disease risk, and how your immune system functions day to day.
This is why subcutaneous fat is sometimes described as an endocrine organ. It’s not passive storage; it’s actively communicating with your brain, liver, heart, and immune system.
Why Medications Are Injected Here
If you’ve ever had an insulin injection or certain vaccines, the needle was likely aimed at your subcutaneous tissue. This layer is the preferred site for medications that need slow, steady absorption into the bloodstream. Because fatty tissue has fewer blood vessels than muscle, drugs injected here enter circulation gradually rather than all at once. That makes it ideal for medications like insulin, blood thinners, and some hormone treatments where a sustained release is more effective than a rapid spike.
Common injection sites, such as the abdomen, outer thigh, and back of the upper arm, are chosen specifically because the subcutaneous layer is thick enough there to absorb the medication reliably.
Conditions That Affect This Layer
Several medical conditions target the subcutaneous tissue specifically. Panniculitis is a group of conditions that cause inflammation in this fat layer, producing tender, painful lumps or nodules under the skin. It has multiple forms depending on the cause. Cold exposure can trigger it, as can bacterial or fungal infections, autoimmune diseases like lupus or Crohn’s disease, reactions to certain medications (including some antibiotics and hormonal contraceptives), and underlying conditions like pancreatic disease.
Erythema nodosum, one of the most common types of panniculitis, typically shows up as red, tender bumps on the shins. Lupus panniculitis causes deeper nodules, often on the face, upper arms, or thighs. Infectious panniculitis develops when bacteria or fungi invade the fat layer directly.
Cellulitis is another condition that can extend into subcutaneous tissue. It starts as a bacterial skin infection but can spread deeper, causing swelling, warmth, and redness that extends beyond the surface. Because the subcutaneous layer connects to surrounding structures, infections here can spread more easily than those confined to the upper skin layers.
Subcutaneous vs. Visceral Fat
Not all body fat is the same, and the distinction matters for your health. Subcutaneous fat is the fat you can pinch between your fingers. Visceral fat, by contrast, sits deeper inside your abdomen, wrapping around your organs. In a healthy person, about 80% of body fat is subcutaneous, with the rest stored as visceral fat and in smaller depots elsewhere.
Subcutaneous fat is generally considered less dangerous than visceral fat. Visceral fat is more metabolically active in harmful ways, producing higher levels of inflammatory molecules and contributing more directly to insulin resistance, heart disease, and type 2 diabetes. That said, excess subcutaneous fat still affects your metabolic health through its adipokine signaling. The location of your fat storage, not just the total amount, shapes your health risk profile.