Thyroid gland

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Thyroid gland

• Position It lies in the front of the neck in
  relation to the larynx, pharynx trachea and
  esophagus.
• Shape The gland consists of right and left lobes
  connected by a narrow isthmus.
• - It is very vascular gland, surrounded by a
  sheath derived from the pretracheal fascia.
• - The fascia attaches the gland to the larynx and
  trachea, so the gland moves up and down with the
  larynx during deglutition.

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Fascial layers of Neck
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Thyroid gland (cont.)

• - Each lobe is pear-shaped with its apex directed
  upward reaching the oblique line on the lamina of
  the thyroid cartilage its base lies below at the
  level of the fifth tracheal ring.
• - The isthmus extends across the midline in front
  of the second, third, and fourth tracheal rings.
• - A pyramidal lobe usually presents extending
  upwards from the isthmus, to the left of the
  midline.
• - A fibromuscular band connects the pyramidal
  lobe to the hyoid bone called levator glandulae
  thyroideae.

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Relations of the lobes

• 1. Anterolaterally (superficial)
• - It is covered by the skin, superficial fascia
  and deep fascia.
• - It is overlapped by the sternothyoid,
  sternohyoid, omohyoid and anterior border of
  sternomastoid muscles.

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Relations of the lobes (cont.)

• 2. Medially
• - It is related to the larynx and pharynx
  above.
• - It is related to the trachea and esophagus
  below.
• - Cricothyroid muscle and its nerve supply, the
  external laryngeal nerve.
• - It is related to the recurrent laryngeal
  nerve (very important) in the groove between the
  trachea and esophagus.

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Relations of the lobes

• 3. Posterolaterally
• - It is related to the carotid sheath with the
  common carotid artery, internal jugular vein, and
  vagus nerve.
• - The posterior border of the lobe is related
  posteriorly to the superior and inferior
  parathyroid glands and the anastomosis between
  the superior and inferior thyroid arteries.

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Relations of the isthmus

• Anteriorly The sternothyroids, sternohyoids,
  anterior jugular veins fascia and skin.
• Posteriorly The second, third, and fourth rings
  of the trachea.
• The terminal branches of the superior thyroid
  arteries anastomose along its upper border.

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Blood supplyA. Arterial supply

• 1. Superior thyroid artery
• - It is a branch of the external carotid artery.
• - It descends to the upper pole of each lobe,
  accompanied by the external laryngeal nerve.

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A. Arterial supply (cont.)

• 2. Inferior thyroid artery
• - It is a branch of the thyrocervical trunk.
• - It ascends behind the gland to the level of the
  cricoid cartilage.
• - It then turns medially and downward, to reach
  the posterior border of the gland.
• - the recurrent laryngeal nerve crosses either in
  front of or behind the artery or may passes
  between its branches.
• 3. Thyroidea ima artery
• - It is an occasional branch may be present.
• - If present it may arise from the
  brachiocephalic artery or the arch of aorta.
• - It ascends in front of the trachea to the
  isthmus.

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B. Venous Lymph drainage

• 1. Superior thyroid vein
• - It drains into the internal jugular vein.
• 2. Middle thyroid vein
• - It drains also into internal jugular vein.
• 3. Inferior thyroid vein
• - It receives its tributaries from the isthmus
  and the lower poles of the gland.
• - The inferior thyroid veins of the two sides
  unite with each other and drain into the left
  brachiocephalic vein.
• Lymph drainage
• The lymph from the thyroid gland drains mainly
  into the deep cervical lymph nodes.
• A few lymph vessels descend to the paratrachial
  nodes.

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Clinical notes

• 1- Movement of the thyroid gland on swallowing
• The gland moves up and down with larynx during
  swallowing because it is enveloped in the
  pretracheal fascia.
• 2- Thyroid gland and the airway
• In enlargement of thyroid gland it compresses the
  trachea because of its intimate relation.
• 3- Retrosternal goiter
• In enlargement of the thyroid gland it descends
  downwards behind the sternum (compressing the
  trachea or veins).
• This is because the attachment of the
  sternothyroids muscles to the thyroid cartilage
  prevents its ascent.
• 4- Thyroid arteries and important nerves
• The superior thyroid artery is related to the
  external laryngeal nerve.
• The terminal branches of the inferior thyroid
  artery are related to the recurrent laryngeal
  nerve.
• In thyroidectomy operations damage to either
  nerve may happen.
• 5- Injury of external laryngeal nerve results in
  paralysis of Cricothyroid muscle and this leads
  to hoarseness of voice and inability to tense
  voice.

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Clinical notes (cont.)

• 6- Injury to recurrent laryngeal nerve produces
• - Unilateral complete injury
• Voice is not greatly affected because of
  compensation of the other normal side.
• -Bilateral complete injury
• Speech is lost (aphonia) and breathing is
  impaired, since the rima glottidis is partially
  closed.
• - Unilateral partial injury
• The abductors of vocal cord are more affected
  resulting in adduction of vocal cords.
• - Bilateral partial injury
• This leads to dyspnea (acute breathlessness) and
  stidor, and cricothyroidotomy or tracheostomy is
  necessary.
• This is because of paralysis of abductors of
  vocal cords on both sides.
• 7- The parathyroid glands and thyroidectomy
• In partial thyroidectomy the posterior part of
  the thyroid gland is left to avoid removal of the
  parathyroid glands which lie on the posterior
  surface of the thyroid gland.

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Nerves related to Thyroid gland
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Parathyroid glands

• - They are four yellowish-brown, ovoid bodies.
• - They are embedded in the fascial capsule of the
  thyroid gland related to its posterior border.
• - They receive blood supply from the superior and
  inferior thyroid arteries.
• The two superior parathyroid glands
• - These are more constant in position.
• - They lie at the level of the middle of the
  posterior border of the thyroid gland.
• The two inferior parathyroid glands
• - They lie close to the inferior poles of the
  thyroid gland.

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Trachea

• It is a cartilaginous and membranous tube. It
  conveys air to and from the lungs. It is about
  10cm in length and is formed of incomplete
  cartilaginous rings completed posteriorly by
  smooth muscles.
• It begins at the lower border of the cricoid
  cartilage opposite the 6th cervical vertebra.
• It ends by dividing into two bronchi opposite the
  lower border of the 4th cervical vertebra.
• Course Its upper half lies in the neck and lower
  half in the thorax. It descends in the midline,
  but at its bifurcation it lies slightly to the
  right.

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Trachea

• Relations in the neck
• Anteriorly skin, fascia, isthmus of the thyroid
  gland (in front of second, third, and fourth
  tracheal rings), inferior thyroid veins, jugular
  arch, hyroidea ima artery (if present), and the
  left brachiocephalic vein in children. It is
  overlapped by the sternomastoid and sternohyoid
  muscles.
• Posteriorly The right and left recurrent
  laryngeal nerves, the esophagus and the vertebral
  column.
• Laterally On each side the trachea is related to
  a lobe of thyroid gland, carotid sheath, and
  inferior thyroid artery.
• Blood supply The trachea is supplied by the
  inferior thyroid arteries in the neck.
• Lymph drainage The lymph vessels drain into the
  pretracheal and paratrachial lymph nodes in the
  neck.
• Nerve supply The vagi, recurrent laryngeal
  nerves and sympathetic trunks.

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Esophagus

• It is a muscular tube about 25cm (10 inches)
  long.
• It begins as a continuation of pharynx at the
  level of cricoid cartilage, opposite the sixth
  cervical vertebrae.
• It ends by joining the cardiac end of the stomach
  opposite the 10th thoracic vertebra.
• Course It descends in the midline then it
  inclines to the left and leaves the neck to the
  thorax.

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Esophagus (cont.)

• Relations in the neck
• Anteriorly The trachea, the recurrent laryngeal
  nerves in the groove between the trachea and
  esophagus.
• Posteriorly The prevertebral fascia, the longus
  colli muscle, and the vertebral column.
• Laterally On each side the trachea is related to
  a lobe of thyroid gland, carotid sheath, and
  inferior thyroid artery. The thoracic duct
  ascends along the left margin for a short
  distance.
• Blood supply The esophagus is supplied by the
  inferior thyroid arteries in the neck.
• Lymph drainage The lymph vessels drain into the
  deep cervical lymph nodes.
• Nerve supply The vagi, recurrent laryngeal
  nerves and sympathetic trunks.

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