H

Hashimoto thyroiditis
A progressive disease of the thyroid gland characterized by the presence of antibodies directed against the thyroid, and by infiltration of the thyroid gland by lymphocytes. Hashimoto thyroiditis is the most common cause of hypothyroidism in North America and Europe. In this condition, the thyroid gland is usually enlarged (goiter) and has a decreased ability to make thyroid hormones. Hashimoto disease predominantly affects women, and can be inherited. It is also known as autoimmune thyroiditis and Hashimoto disease.

Hemoglobin
Hemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues to the lungs. The iron contained in hemoglobin is responsible for the red color of blood.

Histamine
Histamine is a substance in the body that's released during an allergic reaction. It can cause allergy symptoms that affect the eyes, nose, throat, skin, digestive system, and lungs.

Hormone
A chemical substance produced in the body that controls and regulates the activity of certain cells or organs. Many hormones are secreted by specialized glands such as the thyroid gland. Hormones are essential for every activity of daily living, including the processes of digestion, metabolism, growth, reproduction, and mood control. Many hormones, such as the neurotransmitters, are active in more than one physical process.

Hydrogen
The most plentiful element in the universe and one present in all organic compounds. Hydrogen is a gas with an atomic number of 1 and the symbol H.

Hyperglycemia
Glucose (a type of sugar) is the body's main energy source. Hormones, such as insulin, control the level of glucose in the blood. It's unhealthy if a person's glucose levels get too high or too low. Hyperglycemia occurs when the levels get too high. This can happen to someone who has diabetes but doesn't know it yet. It also can happen to someone whose diabetes is not under control. Signs of hyperglycemia include increased urination (peeing), extreme thirst, and unexplained weight loss.

Hypoglycemia
Glucose (a type of sugar) is the body's main energy source. Hormones, such as insulin, control the level of glucose in the blood. It's unhealthy if a person's glucose levels get too high, or too low. Hypoglycemia occurs when the levels get too low. This can happen to people who have diabetes. It can occur if the person doesn't eat enough or if the person takes too much insulin, which lowers glucose levels. A person with hypoglycemia may feel hungry, shaky, sweaty, weak, drowsy, or dizzy. If left untreated, hypoglycemia may even make someone faint or pass out.