Pathophysiology Review
The thyroid gland covers the anterior surfaces of the second to fourth tracheal rings. It comprises a right and left lobe, separated in the middle by an area called the isthmus. It is surrounded with a fibrous capsule. The posterior layer contains four parathyroid glands.
The thyroid consists of loose tissue in the formation of grape-like clusters with many blood vessels. Microscopic analysis reveals follicle walls that are surrounded by cuboidal epithelial cells. The follicle cells produce a glycoprotein called thyroglobulin. With the incorporation of dietary iodine, thyroglobulin is further processed to form a variety of thyroid hormones, most importantly triiodothyronine, commonly referred to a T3, and thyroxine (tetraiodothyronine), commonly referred to as T4.
Hyperthyroid secretion of thyroid hormones results in excessive metabolism. Insufficient secretion of thyroid hormones will result in deficient metabolism. Congenital thyroid insufficiency is manifested by dwarfism and mental retardation.
T3 and T4 Hormones [B]
T3 is produced in small amounts from the thyroid, but, most significantly, it is converted in the liver and kidneys from T4. T4 secretion is controlled by the pituitary secretion of TSH (thyroid-stimulating hormone). Like many other endocrine systems, its regulation works by a negative feedback loop. Increase of TSH will result in an increase formation of thyroxine. A high thyroxine level will cause the TSH to decline.
T3 is the most active thyroid hormone. However, both T3 and T4 have the same function: they increase oxygen availability in virtually all tissues. T3 also functions in maintaining metabolic rate, growth and development.
Parathormone [B]
The parathyroid glands lie posterior to the thyroid. It is a highly vascular tissue that secretes parathormone. Parathormone maintains calcium levels through out the body by stimulating osteoclastic activity (bone breakdown), freeing calcium ions. Reduced parathormone will result in low calcium levels manifesting as muscle stiffness, cramps, spasms, and convulsions.
Up to 30% to 40% of essential hypertension may be due to excessive parathyroid hypertensive factor (PHF), another hormone produced by the parathyroid gland.(1)
