The thyroid gland is located in the front of the neck straddling your windpipe (trachea). It has two side lobes connected by a middle section called the thyroid isthmus.
The thyroid starts developing near the foramen cecum at the posterior one-third of the tongue and descends anteriorly to envelop the larynx via the thyroglossal duct. This duct regresses in most individuals but persists in some as the pyramidal lobe – a typical vestigial thyroid remnant.
Thyroid Lobes
The thyroid gland is a butterfly-shaped organ with two lobes in front of the windpipe (trachea) and larynx. It is a highly vascular gland surrounded by the cricoid and tracheal cartilages and the laryngeal nerve, which innervates the larynx. In addition, you can review the thyroid anatomy first to picture what the thyroid gland looks like.
The superior thyroid artery runs anteriorly to the gland. The thyroid gland secretes the two tropin hormones, T3 and T4, which regulate various metabolic, circulatory, and developmental processes. It divides into ascending and posterior branches that supply the gland’s anterior, lateral, and medial surfaces. The hypothalamus controls the release of these hormones via thyrotropin-releasing hormone (TRH).
On the posterior side of the thyroid, four parathyroid glands, two on each side, are embedded within the connective tissue capsule. They are endocrine glands that produce and secrete parathyroid hormone (PTH). It is the primary regulator of blood calcium levels by increasing calcium excretion.
Several neck muscles course over the thyroid gland, including the omohyoid and sternocleidomastoid muscles. The omohyoid muscle wraps around the hyoid bone and extends inferiorly to the sternum; it limits the superior pole of the thyroid gland from projecting on this structure. The sternohyoideus muscle extends over the thyroid gland and is found on the anterior surface. The superior laryngeal nerve (SLN) runs over the delicate texture of the thyroid gland and is vulnerable to injury when the thyroid is dissected high.
Thyroid Follicles
Like other endocrine glands, the thyroid secretes hormones directly into the blood. However, unlike other glands, the thyroid stores its products in follicles. Each follicle contains a central lumen lined by simple, low-columnar, or cuboidal epithelial cells. Each strand is further separated by septae, formed by invading parts of the surrounding fibrous capsule. In addition to thyrocytes (follicular epithelial cells), each follicle contains a population of calcitonin-secreting C cells.
The thyroid lies below the larynx in a butterfly-shaped space in the neck. It is supported laterally by the lateral thyroid cartilage and posteriorly by the cricoid cartilage. The sternothyroid muscle anchors the thyroid gland in the anterior neck and a fibromuscular structure called the levator glandular thyroidal (abdominal flap).
The thyroid is divided into two lobes and an isthmus, each containing tiny, globular sacs that secrete hormones such as thyroxine and triiodothyronine. These follicles are filled with a glycoprotein-rich fluid called colloid, which includes the prohormone thyroglobulin. During ovulation, these follicles develop into functional thyroid follicles containing a blister where the thyroglobulin is stored. On light microscopy, these vesicles stain brightly with hematoxylin and eosin. The blister contains many mitochondria, rough endoplasmic reticulum, Golgi bodies, and other organelles. In the apex of these cysts are thyrocytes with numerous zonula occludens, zonula adherens, and macula adherents that form the anchoring sites for the thyroglobulin.
Thyroid Hormones
Hormones that control the body’s metabolism are released into the bloodstream by the thyroid gland. It also helps prevent growth and development and supports brain function. Adequate levels of thyroid hormones are required for protein synthesis, especially during pregnancy and childhood. In addition, thyroid hormones upregulate blood vessel receptors, making the body more responsive to catecholamines (epinephrine and norepinephrine) released by the adrenal medulla.
The two lobes of the thyroid are located on either side of the windpipe (trachea) in the front part of the neck. A normal thyroid gland is not visible but may be felt with finger pressure. The thyroid is supplied by superior and inferior thyroid arteries and drained via superior, middle, and inferior thyroid veins.
A complex system of signals controls the amount of thyroid hormones made. The hypothalamus secretes the thyroid-releasing hormone, which stimulates a part of the pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH then activates the thyroid follicular cells to release triiodothyronine (T3) and thyroxine (T4) into the bloodstream.
Both T3 and T4 are tyrosine-based hormones that regulate metabolism. T3 and T4 bind to transport proteins in the blood, and the hormones are released into the body, affecting protein production and other metabolic processes. The cellular machinery that produces thyroid hormones requires adequate amounts of the mineral iodine to function correctly.
Thyroid Nodules
The thyroid gland is a butterfly-shaped endocrine gland in your neck. Its functions include regulating the speed at which your body transforms food into energy for all its cells. The amount of thyroid hormone produced by a thyroid nodule, lump, or bump on the front of your neck may be excessive or insufficient.
Your thyroid has two lobes, the left and right, which are anteriorly connected by a central isthmus, giving it a butterfly-shaped appearance. Occasionally, the thyroid gland can also have a third lobe called a pyramidal lobe, which extends from the isthmus towards the base of your tongue and hyoid bone.
A thin, fibrous band, known as the levator glandular thyroidal, connects the pyramidal lobe to the isthmus of your thyroid. A small percentage of people have an additional artery that supplies the thyroid – the inferior thyroid artery. This artery arises from the thyrocervical trunk, a branch of the subclavian artery.
Thyroid nodules are a common finding. They can be solid or filled with fluid (cystic) and are categorized according to their size:
