Thyroid & Parathyroid Glands

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THYROID  GLAND

  • It’s a midline (Pre-tracheal) structure, located in the anterior region of the neck.
  • It lies against C5-T1 vertebrae.
  • It has 2 lobes (right & left) connected by an isthmus.
  • Isthmus lies against 2nd, 3rd & 4th tracheal rings.
  • Occasionally (33%) a Pyramidal lobe extends upwards from isthmus.

COVERINGS

Inner True Capsule:-

  • It is the condensation of Fibrous stroma

Outer False Capsule:-

  • Derived from Pretracheal fascia
  • Ligament of Berry extends from the false capsule to cricoid cartilage.

Structures Between True & False Capsules:-

  • Parathyroid glands
  • Trunks of blood vessels (Superior & Inferior thyroid vs.)

Deep to the True Capsule:-

  • Venous plexus lies deep to the true capsule (Unlike that of Prostate gland).
  • Thyroidectomy involves ligating the vessels & removing the entire gland.

PRESENTING  PARTS

  • It forms a H-shaped mass
  • Having 2 Lateral lobes & Isthmus

Lateral Lobes:-

  • Each lobe is pyramidal in shape
  • Each lobe measured with-
    • Length 5cm
    • Breadth 3cm
    • Thickness 2cm
  • Each lobe presents-
    • Apex
    • Base
    • 3 Surfaces- Antero-lateral, Postero-lateral, Medial
    • 2 Borders- Anterior, Posterior

Apex-

  • Directed upwards & reaches the oblique line of thyroid cartilage.
  • Sandwiched between the Inferior constrictor medially & Sternothyroid laterally.
  • Superior thyroid artery & External laryngeal nerve diverge near the apex.

Base-

  • Extends up to 5th or 6th tracheal ring.
  • Inferior thyroid artery & Recurrent laryngeal nerve converge near the base.

Antero-Lateral Surface-

  • It is overlapped by- Sternohyoid, Sternothyroid, Sternomastoid & Superior belly of Omohyoid.

Postero-Lateral Surface-

  • It is related with carotid sheath & its contents.

Medial Surface-

It is related with-

  • 2 Tubes- Larynx & Trachea, Pharynx & Oesophagus
  • 2 Muscles- Inferior constrictor & Cricothyroid
  • 2 Nerves- External laryngeal & Recurrent laryngeal nerves

Anterior Border-

  • It separates medial surface from antero-lateral surface.
  • It is related with anterior descending branch of Superior thyroid artery.

Posterior Border-

  • It separates medial surface from postero-lateral surface.
  • It is related with-
    • Anastomosis between Superior & Inferior thyroid arteries
    • Parathyroid glands

Isthmus:-

  • Measures about 1.25cm in vertical & transverse diameters.
  • It lies against 2nd - 4th tracheal rings.
  • It has-
    • 2 Surfaces- Anterior & Posterior
    • 2 Borders- Upper & Lower

Anterior Surface-

It is related with-

  • Skin
  • Superficial fascia
  • Anterior jugular vein
  • Investing layer
  • Sternothyroid
  • Sternohyoid muscles

Posterior Surface-

  • Related with 2nd, 3rd & 4th Tracheal rings.

Upper Border-

It is related with-

  • Anastomosis between the Superior thyroid arteries.
  • Pyramidal lobe (occasional)
  • Levator glanduli thyroidae (a fibro-muscular band)

Lower Border-

It is related with-

  • Inferior thyroid veins
  • Arteria thyroidea ima (occasional)

ARTERIAL  SUPPLY

Superior thyroid artery-

  • Branch of External carotid artery
  • Accompanied by External laryngeal nerve
  • They diverge from each other above the apex of the lobe

Inferior thyroid artery-

  • Branch of Thyro-cervical trunk
  • Accompanied by Recurrent laryngeal nerve
  • They converge towards the base of the lobe

Arteria thyroidea ima (occasional)- 

  • Branch of Brachio-cephalic trunk / Arch of aorta

Accessory thyroid arteries

VENOUS  DRAINAGE

  • Veins do not follow the arteries.
  • They arise from the venous plexus deep to the true capsule.
They are drained by-
  • Superior thyroid veins- drain into Internal jugular vein
  • Middle thyroid veins- drain into Internal jugular vein
  • Inferior thyroid veins- drain into Brachiocephalic vein
  • 4th thyroid vein (Kocher’s vein)- drains into Internal jugular vein

LYMPHATIC  DRAINAGE

  • Lymphatics form plexus around the thyroid follicles.
  • From the plexus lymph vessels run in 2 groups -
    • Upper group of lymph vessels
    • Lower group of lymph vessels
  • Upper group drains into Prelaryngeal & Jugulo-digastric lymphnodes
  • Lower group drains into Pretracheal lymphnodes

NERVE  SUPPLY

Sympathetic Fibres-

  • Vaso-motor in function.
  • Derived from superior, middle & inferior cervical ganglia.

Parasympathetic Fibres-

  • Derived from Vagus & Recurrent laryngeal nerves.

DEVELOPMENT

  • Thyroid gland develops as an Endodermal diverticulum from the floor of primitive pharynx.
  • The diverticulum forms Thyro-glossal duct.
  • It passes through the substance of the tongue, in front of the hyoid bone.
  • In front of the trachea the duct forms a bilobed mass.
  • Duct disappears later, its cephalic end remains as Foramen caecum & caudal end forms the Pyramidal lobe.

APPLIED  ASPECTS

Goitre:-

  • Enlargement of thyroid gland is called Goitre.
  • May be due to hypo function or hyper function.
  • Grossly enlarged or Nodular.
Hyperthyroidism / Thyrotoxicosis
  • Feature- Exophthalmia
Hypothyroidism
  • Features- Myxoedema, Cretinism

Thyroid Tumors:-

  • Benign Adenoma (Thyroid cyst)
  • Malignant growth
  • Invades the neighboring structures & produces pressure symptoms.

Thyroglossal Cyst:-

  • Congenital anomaly
  • Typical cystic appearance
  • Non-closure of Thyroglossal duct
  • Can be seen anywhere from the foramen caecum to thyroid gland.
  • Ultrasound useful in differentiating cystic from complex or solid masses.

Thyroglossal Fistula:-

  • A congenital anomaly secondary to thyroglossal cyst.
  • Cyst is connected to the external surface (skin), through the fistula.
  • The fistula will be seen as a discharging orifice in the midline of the neck.

Ectopic Thyroid:-

  • A Congenital anomaly
  • Gland is found in the posterior triangle of the neck or retro-sternal region.

Lingual Thyroid:-

  • A Congenital anomaly
  • Failure of caudal migration of thyroid rudiment.

Accessory Thyroid:-

  • A Congenital anomaly
  • Nodules of thyroid tissue are found around the main gland.

 


PARATHYROID  GLANDS

  • Parathyroid glands are small, yellowish-brown endocrine glands.
  • They are situated along the posterior border of pyramidal lobes of thyroid gland.
  • They are four in number and arranged as superior and inferior pairs.
  • They secrete parathyroid hormone.

SUPERIOR  PARATHYROID  GLAND

  • It usually lies between the true and false capsules of thyroid gland.
  • It is constant in position at the level of cricoid cartilage.
  • It lies dorsal to the recurrent laryngeal nerve.

INFERIOR  PARATHYROID  GLAND

  • It occupies one of the three variable positions –
  • In the lower pole of thyroid gland, below the loop of inferior thyroid artery and within the false capsule.
  • In the lower pole of thyroid gland, above the loop of inferior thyroid artery and outside the false capsule.
  • In the lower pole of thyroid gland, beneath the true capsule and within the substance of thyroid gland.

BLOOD  SUPPLY

  • The parathyroids are mostly supplied by inferior thyroid artery.
  • From the anastomosis between superior and inferior thyroid arteries.
  • Venous blood and lymphatics accompany those of thymus and thyroid gland.

NERVE  SUPPLY

  • The sympathetic nerves are derived from the superior and middle cervical ganglia of sympathetic trunks.
  • They convey mainly vaso-motor fibres to the gland.
  • The secretory activities of the glands are controlled by concentration of calcium level in the blood. (low level of calcium stimulates and high level inhibits the glands)

DEVELOPMENT

  • Superior parathyroids are developed from endoderm of fourth pharyngeal pouch (Parathyroid IV).
  • Inferior parathyroids are developed from endoderm of third pharyngeal pouch in common with the thymus (Parathyroid III).
  • Due to caudal migration of the thymus, the parathyroid III occupies the lower position than parathyroid IV.

APPLIED  ASPECTS

Tumours of Parathyroid Gland :-

  • Tumours lead to hypersecretion of parathyroids.
  • Hyperparathyroidism is characterized by generalized osteitis fibrosa.
  • It makes the bone soft due to decalcification, hypercalcaemia and formation of stones in the kidney.

Tetany :-

  • It is a condition of hypoparathyroidism.
  • It is characterized by hypocalcaemia and increased neuromuscular excitability causing convulsions and carpopedal spasms.


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