THYROID AND PARATHYROID

Thyroid and Parathyroid Glands

Introduction

This section presents an overview of the thyroid and parathyroid glands, including their anatomy, functions, blood supply, and clinical relevance, as outlined by Dr. A.U. Agu. The discussion is segmented into two main parts: the thyroid gland and parathyroid glands, including specific subtopics under each gland.

Thyroid Gland

Overview

The thyroid gland is recognized as an endocrine gland characterized by its brownish-red coloration and high vascularity. It is situated anteriorly in the lower neck, spanning cervical vertebrae levels C5 to T1. The thyroid is ensheathed by the pretracheal layer of deep cervical fascia.

The gland is comprised of two lobes, right (Rt) and left (Lt), connected by a narrow median isthmus. The weight of the thyroid gland averages around 25 grams but varies, being slightly heavier in females and subject to enlargement during menstruation and pregnancy. Notably, there is no significant weight difference between genders from 8 months to 15 years.

Anatomy
Lobes
  • The thyroid lobes are approximately conical in shape, reaching ascending apices at the level of the laminae of the thyroid cartilage and bases at the level of the 4th or 5th tracheal cartilages.
  • Each lobe typically measures about 5 cm in length, with its greatest transverse and anteroposterior extents being 3 cm and 2 cm, respectively.
  • The posteromedial aspects of the lobes attach to the side of the cricoid cartilage via a lateral thyroid ligament.
Isthmus
  • The isthmus connects the lower parts of the two lobes, sometimes being absent.
  • It measures approximately 1.25 cm in both transverse and vertical dimensions and is usually positioned anterior to the 2nd and 3rd tracheal cartilages, with substantial variability.
Pyramidal Lobe
  • The pyramidal lobe exhibits a conical shape that often ascends towards the hyoid bone from the isthmus or adjacent parts of either lobe (more frequently the left lobe).
  • A fibrous or fibromuscular band, known as the levator of the thyroid gland, may extend from the body of the hyoid to the isthmus or pyramidal lobe.
Capsule of the Thyroid Gland
  1. True Capsule: This layer is a peripheral condensation of connective tissue from the gland itself.
  2. False Capsule: Derived from the pretracheal layer of deep cervical fascia.
Surfaces and Relations
Convex Lateral (Superficial) Surface
  • Covered by muscles including sternothyroid, while more anteriorly, it is covered by sternohyoid and the superior belly of omohyoid.
  • The inferior aspect is overlapped by the anterior border of the sternocleidomastoid muscle.
Medial Surface
  • Related to two tubes: the trachea and the esophagus.
  • Associated with two muscles: the inferior pharyngeal constrictor and the posterior part of the cricothyroid.
  • Contains two nerves: the external laryngeal nerve and the recurrent laryngeal nerve.
Posterolateral Surface
  • Relates to the carotid sheath and overlaps the common carotid artery.
Borders
  • Anterior Border: Thin and related to the anterior branch of the superior thyroid artery.
  • Posterior Border: Rounded and related to the inferior thyroid artery, anastomosing with the posterior branch of the superior thyroid artery, as well as the thoracic duct and the parathyroid gland.
Isthmus Specifics
  • Has two surfaces: anterior and posterior, as well as two borders: superior and inferior.
  • The anterior surface is covered by the right and left sternothyroid and sternohyoid muscles, anterior jugular veins, fascia, and skin.
  • The posterior surface corresponds to the 2nd to 4th tracheal rings, with specific anatomical relationships to the superior and inferior thyroid arteries and veins.
Blood Supply
  • Arterial Supply:
    • Supplied by the superior thyroid artery (1st anterior branch of the external carotid artery) and the inferior thyroid artery (from the thyrocervical trunk, a branch of the subclavian artery).
    • Occasionally, an additional artery, the arteria thyroidea ima, arises from the brachiocephalic trunk or aortic arch.
  • Venous Drainage:
    • Primarily through the superior and middle thyroid veins into the internal jugular vein (IJV).
    • The inferior thyroid vein drains into the brachiocephalic vein.
Lymphatics
  • Lymphatic vessels of the thyroid communicate with the tracheal plexus and pass to varying lymph nodes, including:
    • Prelaryngeal nodes above the thyroid isthmus.
    • Pretracheal nodes.
    • Paratracheal nodes.
  • Some lymphatics may drain directly to the thoracic duct with no intervening nodes.
Innervation
  • Innervation comes from the superior, middle, and inferior cervical sympathetic ganglia.
Applied Anatomy
  • Clinical considerations relevant to the thyroid gland include:
    • Thyroglossal duct cyst.
    • Ectopic thyroid gland (for example, lingual thyroid).
    • Accessory thyroid glandular tissue.
    • Thyroidectomy procedures.
    • Goiter, which refers to the swelling of the thyroid gland.
Other Thyroid Conditions
  1. Goiter:
    • Thyroid swelling that can be harmless and may arise from iodine deficiency or Hashimoto’s thyroiditis.
  2. Thyroiditis:
    • Inflammation of the thyroid, often due to viral infection or autoimmune conditions, which may present as either painful or asymptomatic.
  3. Hyperthyroidism:
    • Characterized by excessive production of thyroid hormone, frequently caused by Graves' disease or overactive thyroid nodules.
  4. Hypothyroidism:
    • Inadequate production of thyroid hormones primarily resulting from autoimmune diseases.
  5. Graves' Disease:
    • An autoimmune disease resulting in overstimulation of the thyroid, leading to hyperthyroidism.
  6. Thyroid Cancer:
    • Generally uncommon, treatable via surgery, radiation, and hormonal therapy.
  7. Thyroid Nodules:
    • Small abnormal lumps in the thyroid gland; while the majority are benign, a few can be cancerous and may secrete excess hormones, causing hyperthyroidism.
  8. Thyroid Storm:
    • A rare but severe form of hyperthyroidism where extremely high thyroid hormone levels induce critical illness.

Parathyroid Gland (PTG)

Overview

The parathyroid glands are small endocrine glands that possess a yellowish-brown color, characterized by an ovoid or lentiform shape. They are typically located between the posterior borders of the thyroid gland and its capsule. Each gland measures approximately 6 x 3–4 x 1–2 mm (average: 6 x 4 x 2 mm) and weighs around 50 mg, with a common configuration of typically two glands per side (superior and inferior), although there can be more or less.

Identification
  • The identification of PTGs is facilitated by the anastomotic connections between the superior thyroid artery (STA) and inferior thyroid artery (ITA) along the posterior border of the thyroid gland.
  • The superior parathyroid gland is usually designated as PT IV, while the inferior parathyroid gland is PT III.
Position Variability
  • PT IV (superior) is more consistent in its position, generally found at the middle of the posterior border of the lobe of the thyroid gland.
  • PT III (inferior) exhibits variability in position, which may include locations:
    • Within the thyroid gland capsule, below the inferior thyroid artery.
    • Behind and outside the capsule above the ITA.
    • Within the lobe's substance itself.
Functions
  • The primary function of the parathyroid glands is to secrete parathyroid hormone (PTH). This hormone plays a crucial role in regulating calcium (Ca) and phosphorus levels in the body, maintaining homeostasis and physiological functions.
Vascular Supply
  • Both superior and inferior parathyroid glands are generally supplied by the inferior thyroid artery.
  • The superior parathyroid glands may also receive blood supply from the superior thyroid artery or anastomoses between the STA and ITA in about 10–15% of cases.
  • Venous drainage occurs into the plexus of veins located on the anterior surface of the thyroid gland.
  • Parathyroid glands have numerous lymph vessels that are associated with those of the thyroid and thymus glands.
Innervation
  • Parathyroid innervation is sympathetic, coming either directly from the superior or middle cervical ganglia or through a plexus located in the fascia on the posterior surface of the gland.
Disorders of the Parathyroid Gland
  • Common disorders include hyperparathyroidism and hypoparathyroidism, which reflect dysfunction in hormone regulation impacting calcium metabolism and overall endocrine health.

Conclusion

The thyroid and parathyroid glands play vital roles in various physiological processes, largely influencing metabolism, growth, and calcium homeostasis in the body. As endocrine organs, understanding their anatomy, functions, blood supply, and the disorders associated with them is essential for clinical practice and health care management.