27.1: Introduction to the Thyroid and Parathyroid
By the end of this section, you should be able to:
- 27.1.1 Describe the function of the thyroid and parathyroid glands.
- 27.1.2 Discuss hormones associated with the thyroid and parathyroid glands.
Thyroid and Parathyroid Glands
The thyroid gland is a butterfly-shaped gland located at the front of the neck below the voice box (see Figure 27.2). This important gland regulates metabolism in the body through the production and secretion of thyroid hormones, which require iodine for synthesis. The thyroid gland produces the active thyroid hormone, triiodothyronine (T3) , and the inactive thyroid hormone, thyroxine (T4) . Thyroid follicular cells secrete T3 and T4, while parafollicular cells secrete calcitonin cells that aid calcium metabolism. Follicular cells line the follicles, which are small spherical structures in the thyroid gland. They absorb iodine from the blood and combine it with T3 and T4. Parafollicular cells are scattered between the follicles of the thyroid gland. Their main function is to secrete calcitonin for calcium regulation. Calcitonin lowers the levels of calcium in the blood by inhibiting the activity of osteoclasts—cells that break down bone tissue and release calcium into the blood. When calcium levels in the blood rise, the thyroid gland releases calcitonin, which binds to receptors on the surface of osteoclasts and inhibits their activity, reducing the amount of calcium released from the bone into the blood, thereby lowering calcium levels in the blood (McLaughlin & Jialal, 2022).
Link to Learning
Diagnostic Testing
Diagnostic testing is key in determining thyroid and parathyroid conditions. Review the article Thyroid Function Testing in the Diagnosis and Monitoring of Thyroid Function Disorder. The first figure in the article illustrates an algorithm commonly used when diagnosing thyroid and parathyroid disorders.
Other diagnostic tests that assist in diagnosing thyroid and parathyroid conditions include:
- Ionized serum calcium: This test is performed for conditions that affect the body’s ability to balance the amounts of ionized calcium and bound calcium in the blood. The normal range is 1.12–1.23 mmol/L, although ranges may vary slightly depending on the laboratory (American Board of Internal Medicine, 2023).
- Vitamin D (calciferol): This test is performed to determine the amount of vitamin D in the blood, which helps the body to absorb calcium. The normal range is 30–60 ng/mL, although ranges may vary slightly depending on the laboratory (American Board of Internal Medicine, 2023).