- The thyroid gland is a vital endocrine organ located in the anterior neck, just below the larynx (voice box) and in front of the trachea (windpipe).
- Shaped like a butterfly, it consists of two lateral lobes connected by a thin tissue called the isthmus. In some individuals, a third small lobe called the pyramidal lobe may extend upward from the isthmus. Though small in size, the thyroid gland plays an essential role in regulating metabolism, growth, and development through the production of thyroid hormones.
- The primary function of the thyroid gland is the synthesis and release of two major hormones: thyroxine (T4) and triiodothyronine (T3). These hormones are made from the amino acid tyrosine and the element iodine, which is actively taken up from the bloodstream by thyroid cells. Once synthesized, T3 and T4 are stored in the gland’s follicular cells, which surround colloid-filled follicles. When stimulated, typically by thyroid-stimulating hormone (TSH) from the anterior pituitary gland, the thyroid releases T3 and T4 into the bloodstream. These hormones then bind to receptors in nearly every tissue in the body to regulate metabolic rate, oxygen consumption, heart rate, and thermogenesis.
- T3 is the more biologically active of the two hormones, though T4 is produced in greater quantity. Much of the T4 released is converted into T3 in peripheral tissues such as the liver and kidneys. The secretion of T3 and T4 is controlled by a negative feedback loop involving the hypothalamus–pituitary–thyroid (HPT) axis. When circulating thyroid hormone levels are adequate, they inhibit the release of thyrotropin-releasing hormone (TRH) from the hypothalamus and TSH from the pituitary, thereby maintaining hormonal balance.
- In addition to T3 and T4, the thyroid gland also produces calcitonin, a hormone secreted by parafollicular cells (C cells). Calcitonin plays a lesser but still important role in regulating calcium homeostasis by lowering blood calcium levels, mainly by inhibiting the activity of osteoclasts in bones.
- Thyroid function is essential for normal growth and neurological development, especially in infancy and childhood. A deficiency of thyroid hormones during critical developmental periods can lead to cretinism, a condition marked by severe physical and cognitive impairment. In adults, thyroid dysfunction manifests as either hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). Hypothyroidism can cause fatigue, weight gain, cold intolerance, and depression, while hyperthyroidism may lead to weight loss, anxiety, heat intolerance, and palpitations. Common thyroid disorders include Hashimoto’s thyroiditis, Graves’ disease, goiter, and thyroid nodules or cancer.
- The thyroid gland is commonly evaluated through clinical examination, hormone level testing (TSH, free T4, and T3), ultrasound imaging, and sometimes fine needle aspiration biopsy. Treatment for thyroid disorders varies depending on the condition and may include hormone replacement therapy, antithyroid medications, radioactive iodine therapy, or surgical removal of the gland.
