Thyroid nodules, Causes:  toxic multinodular goiter, as defined by Mayo Clinc

Thyroid nodules, Causes:

By Mayo Clinic staff

Your thyroid gland consists of two lobes that resemble the wings of a butterfly. The thyroid takes up iodine from food you eat and uses it to manufacture two main hormones, thyroxine (T4) and triiodothyronine (T3). These hormones maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate, and help regulate the production of protein. Your thyroid gland also produces calcitonin, a hormone that regulates the amount of calcium in your blood.
It's not known what causes most nodules to develop, but the following factors are sometimes involved:
• A lack of iodine in your diet
• Hashimoto's disease, an autoimmune disorder that causes chronic inflammation — and underactivity (hypothyroidism) — of your thyroid gland
• A genetic defect of receptors that release thyroid-stimulating hormone (TSH)
• Radiation treatments to your head or neck in childhood
Several types of nodules can develop in your thyroid gland:
• Colloid nodule. Most thyroid nodules are colloid nodules — noncancerous (benign) overgrowths of normal thyroid tissue. You may have just one colloid nodule or many. Although these nodules may grow larger, they don't spread beyond your thyroid gland.
• Follicular adenoma. This type of nodule also is benign.
• Thyroid cyst. These fluid-filled areas of the thyroid can range in size from less than 1/3-inch (about 8 millimeters) in diameter to 1 inch (25 millimeters) or more. Many thyroid cysts are entirely filled with fluid, but some cysts, called complex cysts, also have solid components. Fluid-filled cysts are usually benign, but complex cysts are sometimes malignant.
• Inflammatory nodule. This occasionally develops as a result of chronic inflammation of your thyroid gland (thyroiditis). One rare type of thyroiditis — subacute thyroiditis — causes severe pain in the thyroid gland. Other types are painless and sometimes occur after pregnancy (postpartum thyroiditis).
• Thyroid cancer. Although the chances that a nodule is malignant are small, you're at higher risk if you have a family history of thyroid or other endocrine cancers, are younger than 30 or older than 60, are a man, or have a history of head or neck radiation. Malignant nodules are usually large and hard and may cause neck discomfort or pain.
• Multinodular goiter. "Goiter" is a term used to describe any enlargement of the thyroid gland. Several factors can lead to a goiter, including the presence of a number of thyroid nodules. This condition, called multinodular goiter, can cause a tight feeling in your throat and difficulty breathing or swallowing.
• Hyperfunctioning thyroid nodule (toxic adenoma, toxic multinodular goiter, Plummer's disease). These nodules grow and produce thyroid hormones independent of the influence of TSH, a substance released by the pituitary gland, which normally regulates the production of your thyroid hormones.
Hyperfunctioning thyroid nodules cause high blood levels of thyroxine along with low or nonexistent levels of TSH. A genetic defect of the TSH receptors may play a role in the overactivity of these nodules.