The thyroid gland is located on the front part of the neck below the thyroid cartilage (Adam's apple). The gland produces thyroid hormones, which regulate body metabolism. Thyroid hormones are important in regulating body energy, the body's use of other hormones and vitamins, and the growth and maturation of body tissues.
Diseases of the thyroid gland can result in either production of too much (overactive thyroid disease or hyperthyroidism), too little (underactive thyroid disease or hypothyroidism) thyroid hormone,thyroid nodules, and/or goiter. All types of thyroid problems in women are much more common than thyroid problems in men.
- Production of thyroid hormones: The process of hormone synthesis begins in a part of the brain called the hypothalamus. The hypothalamus releases thyrotropin-releasing hormone (TRH). The TRH travels through the venous plexus located in the pituitary stalk to the pituitary gland, also in the brain. In response, the pituitary gland then releases thyroid-stimulating hormone (TSH) into the blood. The TSH travels to the thyroid gland and stimulates the thyroid to produce the two thyroid hormones, L-thyroxine (T4) and triiodothyronine (T3). The thyroid gland also needs adequate amounts of dietary iodine to be able to produce T4 and T3.
- Regulation of thyroid hormone production: To prevent the overproduction or underproduction of thyroid hormones, the pituitary gland can sense how much hormone is in the blood and adjust the production of hormones accordingly. For example, when there is too much thyroid hormone in the blood, TRH and TSH production are both decreased. The sum effect of this is to decrease the amount of TSH released from the pituitary gland and to reduce production of thyroid hormones from the thyroid gland to restore the amount of thyroid hormone in the blood to normal. Defects in these regulatory pathways may result in hypothyroidism (underactive thyroid problem) or hyperthyroidism (overactive thyroid problem).
- Thyroid goiter: Thyroid goiter is any enlargement of the thyroid that can occur with hyperthyroidism or hypothyroidism but also with benign and malignant (cancerous) nodules. Worldwide, the most common cause of goiter is iodine deficiency. Although it used to be very common in the U.S., it is now less common with the use of iodized salt. Multiple nodules in the thyroid are very common, but only about 5% of the nodules are a thyroid cancer. Thyroid cancer is diagnosed after a thyroid ultrasound exam and a needle aspiration biopsy of the nodule.