Thyroid Gland Anatomy

Thyroid Gland

• Located between C5 and T1.
• Situated between the oblique line and the 4th-5th tracheal rings.
• Composed of 2 lobes connected by an isthmus.
• Measures approximately 3cm in length, 2cm in depth, and 5cm in height.
• Weighs about 25 g.

Lateral and Medial Relations of the Thyroid

• Lateral relations include the sternothyroid, sternohyoid, omohyoid (superior belly), and sternocleidomastoid (SCM) muscles.
• Medial relations include the larynx, trachea, esophagus, and the recurrent and external laryngeal nerves.
• The carotid sheath is posterolateral to the thyroid.

Vessels and Nerves of the Thyroid

• The superior thyroid artery (branch of external carotid artery) and the inferior thyroid artery (branch of thyrocervical trunk) supply blood to the thyroid.
• The superior and middle thyroid veins drain into the internal jugular vein, while the inferior thyroid vein drains into the brachiocephalic vein.
• The superior, middle, and inferior cervical sympathetic ganglia, as well as the vagus nerve, provide innervation.
• Lymphatic drainage occurs through periglandular, prelaryngeal, pretracheal, and paratracheal nodes, flowing into the mediastinal lymph nodes.

Thyroid Hormones and Function

• Thyroid hormones regulate metabolism.
• The main hormones are T3 (tri-iodothyronine) and T4 (thyroxine).
• Calcitonin is also produced, which helps regulate blood calcium levels.
• A goiter, or bulge in the neck, can be caused by hyperthyroidism or iodine deficiency.
• Hyperthyroidism is caused by excessive thyroid hormone, leading to sensitivity to heat, hyperactivity, and excessive eating.

Parathyroid Glands

• Secrete parathyroid hormone to regulate calcium levels.
• Calcium is crucial for muscle contraction and nerve conduction.
• Each gland measures about 3x2x5 mm and weighs approximately 50 mg.
• Cervical ganglia innervate the thyroid and parathyroid glands, excluding the inferior ganglia for the parathyroid.

Suprarenal Glands

• Small, yellowish bodies that are flattened anteroposteriorly.
• Located anterosuperior to the superior renal pole.
• Surrounded by connective tissue containing perinephric fat and enclosed in renal fascia.
• Separated from the kidneys by fibrous tissue.

Characteristics of Suprarenal Glands

• The right gland is irregular tetrahedron shaped.
• The left gland is semilunar, larger, and situated superiorly.
• In adults, they measure approximately 5cm x 3cm x 1cm, with the medulla accounting for 1/10th of the total weight (5g).

Relations of the Right Suprarenal Gland

• The right gland has a posterior relationship to the inferior vena cava and the right hepatic lobe.
• Anteriorly, it is related to the diaphragm and the superior pole of the right kidney.
• The base is inferiorly positioned.
• The base adjoins the anteromedial aspect of the right superior renal pole.
• The base overlaps the upper part of the kidney's medial border.
• The anterior surface features a medial, narrow, vertical area uncovered by peritoneum (posterior to the inferior vena cava).
• The lateral triangular area on the anterior surface is in contact with the liver.
• The upper part of the lateral triangular area is devoid of peritoneum.
• The upper part is in contact with the bare area of the liver.
• The inferior part of the anterior surface is covered by the peritoneum.
• The peritoneum is reflected onto it from the coronary ligament.
• The inferior part is overlapped by the duodenum.

The Hilum of the Right Suprarenal Gland

• Located on the anterior surface.
• The right suprarenal vein emerges, and the superior, middle, and inferior suprarenal arteries enter.

Posterior Surface and Medial Border of the Right Suprarenal Gland

• The upper area of the posterior surface is convex and related to the diaphragm.
• The lower area is concave and contacts the superior pole and adjacent anterior surface of the kidney.
• The medial border is related to the celiac ganglion and inferior phrenic artery.

Lateral and Medial Characteristics of the Left Suprarenal Gland

• The left suprarenal gland has a medially convex shape and a laterally concave shape.
• The superior border is sharp, while the inferior border is rounded.
• The superior area of the anterior surface is covered by the peritoneum of the omental bursa.
• The inferior area is not covered by peritoneum where the suprarenal gland is in contact with the pancreas and splenic artery.

Hilum, Posterior Surface, and Medial Border of the Left Suprarenal Gland

• The hilum faces ventrocaudally
• The left superior, middle, and inferior suprarenal arteries enter, and the suprarenal vein emerges at the lower end.
• The lateral area adjoins the left kidney, and the medial area is related to the diaphragm's left crus.
• The medial border is associated with the celiac ganglion, inferior phrenic artery, and left gastric artery.

Suprarenal Gland Structure

• The suprarenal gland has 2 zones: the cortex and the medulla.
• The cortex is the outer, yellow zone and forms the main mass of the gland.
• The medulla is the inner zone, making up 1/10 of the gland volume, and is dark red or pearly gray.
• Chromaffin cells synthesize and expel noradrenaline and adrenaline.

Vasculature and Lymphatics of the Suprarenal Glands

• The superior suprarenal artery (inferior phrenic artery), middle suprarenal artery (abdominal aorta), and inferior suprarenal artery (renal artery) supply blood to the suprarenal glands.
• Right suprarenal vein > inferior vena cava.
• Left suprarenal vein > renal vein.
• Drainage occurs through lateral aortic lymph nodes.

Nerves of the Suprarenal Glands

• Symphathetic innervation: Cervicothoracic (stellate) ganglion.
• Parasympathetic innervation: Vagus nerve.
• Myelinated preganglionic sympathetic fibers are distributed to medullary chromaffin cells.
• The cortex is under the control of adrenocorticotropic hormone (ACTH).

Thymus Gland

• The thymus is one of the two primary lymphoid organs.
• The thymus has responsibility for providing T lymphocytes to the whole body.
• Appearance varies with age, reaching its maximum size up to age 15.

Thymus Features and Relations

• It is a soft bilobed organ.
• The two parts are joined by connective tissue, which merges with the capsule of each lobe.
• Early in life, it is deep red; it becomes thinner and grayer with age, and is yellow in late life as adipose tissue.
• It weighs 10-15 g at birth and rapidly increases to 20 g.
• It lies in the superior and anterior inferior mediastinum.
• Anteriorly, it is related to the sternum and the upper 4 costal cartilages, sternohyoid and sternothyroid muscles.
• The lower border is at the 4th costal cartilage level.

Posterior Relations, Vessels, and Nerves of the Thymus

• Posteriorly, the thymus relates to the pericardium, aortic arch, the left brachiocephalic vein, and the front and sides of the trachea.
• Arterial supply is from the internal thoracic artery and the inferior thyroid artery.
• Venous drainage is via the left brachiocephalic, internal thoracic, and inferior thyroid veins.
• Lymphatic drainage: No afferent lymphatics.
• Efferent lymph vessels originate from the medulla/corticomedullary junction and drain into related regional nodes.
• Sympathetic innervation: Cervicothoracic (stellate) ganglion.
• Parasympathetic innervation: Vagus nerve.