What Is Glutamic Acid Decarboxylase (GAD)?

Glutamic acid decarboxylase (GAD) is an enzyme that starts the formation of gamma-aminobutyric acid (GABA). GABA is a chemical in the brain and other areas of the body that limits the body's ability to send and receive messages.

This article will cover what GAD is and how it relates to diabetes and other conditions. It will also discuss the importance of GAD autoantibody testing.

Image of an enzyme


Glutamic Acid Decarboxylase (GAD) Antibodies

GAD is an enzyme. Enzymes are proteins that increase the speed of a chemical reaction in the body (because enzymes can be found in extracellular space as well as intracellular space).

The GAD enzyme is primarily produced by the islet cells of the pancreas. It is the enzyme responsible for forming GABA.

GABA is a neurotransmitter that inhibits or reduces the body's ability to send and receive signals in the brain and spinal cord.

GAD research is focused on how GAD autoantibodies affect the body. Antibodies attack foreign cells in the body. However, autoantibodies mistakenly attack healthy cells. GAD autoantibodies attack the pancreas, which can lead to certain conditions like type 1 diabetes.

GAD and Diabetes

GAD autoantibodies attack healthy pancreas cells, injuring the pancreas. When this happens, the pancreas does not make enough insulin to support the body's needs. This can lead to the development of type 1 or latent autoimmune diabetes in adults (LADA).

Type 1 diabetes and LADA are not only caused by GAD autoantibodies. Other autoantibodies can cause the development of these conditions. They include:

  • Islet cell cytoplasmic autoantibodies (ICA)
  • Insulinoma-associated-2 autoantibodies (IA-2A)
  • Insulin autoantibodies (IAA)

Healthcare providers may find determining whether someone has type 2 or type 1 diabetes difficult. They are treated differently, and it can become very dangerous if they are not treated correctly. A GAD autoantibody test helps healthcare providers single out which type of diabetes someone has to allow for proper treatment.

GAD Autoantibodies and Other Conditions

GAD is needed to form GABA, a neurotransmitter found in the brain. When someone has GAD autoantibodies, they are not able to produce normal levels of GABA. This can result in cognitive and motor changes in the body.

Neurological conditions that have been associated with GAD autoantibodies include:

GAD Antibody Tests

A GAD antibody test looks for antibodies that attack the normal GAD cells. If someone is positive for GAD antibodies, they are at a higher risk for developing type 1 diabetes.

People with stiff-person syndrome will have extremely high GAD antibody levels. GAD levels can be 10 times above normal levels in those with diabetes. But in those with stiff person syndrome, the levels are 10 times higher than those seen in diabetes or may be found in the spinal fluid.


GAD is an enzyme that contributes to the neurotransmitter called GABA. GABA plays an important role in the brain and other areas of the body. The body can produce GAD autoantibodies, which attack healthy GAD cells. GAD autoantibodies have been linked to type 1 diabetes, LADA, and neurological conditions like stiff person syndrome.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Lernmark A. Glutamic acid decarboxylase – gene to antigen to diseaseJ Intern Med. 1996;240(5):259-277.

  2. University of Rochester Medical Center. Diabetes autoantibody panel.

  3. Tohid H. Anti-glutamic acid decarboxylase antibody positive neurological syndromesNeurosciences (Riyadh). 2016;21(3):215-222. doi:10.17712/nsj.2016.3.20150596

  4. National Institute of Neurological Disorders and Stroke. Stiff-person syndrome.

By Patty Weasler, RN, BSN
Patty is a registered nurse with over a decade of experience in pediatric critical care. Her passion is writing health and wellness content that anyone can understand and use.