Download Thyroid gland (level 3 discussion)

Agodirin SO. The thyroid gland.medimag.com.ng vol1 level3
Thyroid gland (level 3 discussion)
Introduction
This volume begins the discussion of the thyroid gland. The thyroid gland is the
endocrine organ that is most frequently operated upon by the surgeon. In this level 3
discussion, the reader should expect to find the theoretically aspects of the discussion
immediately followed by the clinical relevance because the aim of knowing about the
theory of any organ or disease is to apply it clinically. We hope this volume serves that
purpose.
As is the policy, the discussion will progress in small volumes. The volumes will address
the diseases of thyroid gland from the basics to advanced discussions. This introductive
volume cu will address the Anatomy, embryogenesis, surface anatomy and dimensions of
the gland
Embryogenesis
The thyroid gland is an endocrine organ. It is the earliest endocrine organ to develop at
embryogenesis. Its development is influenced by chromosome 22. It develops from the
floor of the pharynx by the proliferation of the epithelium (The epithelium here is
appropriately referred to as the endodermal lining or endoderm) of the pharynx. In the
adult individual, the point where the thyroid gland develops from is marked by a
depression called the foramen caecum. This is at the junction of the anterior two thirds
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Agodirin SO. The thyroid gland.medimag.com.ng vol1 level3
and posterior third of the tongue. This is also the point between the tuberculum impar
and the copula
After proliferation of the endodermal, the proliferated tissue which is the anlarge of the
developing thyroid gland invaginates and descends anterior to the developing pharynx,
the hyoid bone and the trachea to its definitive position which is anterior to the airway,
specifically anterior to the voice box/ larynx (more specifically the thyroid and cricoid
cartilages and the upper tracheal cartilaginous rings).
Formation of the lateral lobes
During its downward migration it becomes bilobed when it approaches the definitive
position or when it reaches the definitive position. The formation of the lateral lobe is
described by two theories. The first describes lateral migration of the fully descended
anlarge, while the second describes failure of failure of lateral separately formed anlarge
with the fully descended midline anlarge. In accordance with the former theory, it is
believed that early lateral migration of the developing anlarge above the level of the
hyoid bone may lead to migration of each lateral lobe separately with congenital absence
of the isthmus.(theory proposed for isthmus agenesis).
Migration of the thyoid anlarge to its definitive position
The descent of the migrating gland may not be straight forward, its anlarge may roam or
dance around the body of the hyoid bone; it may initially descend behind the bone then
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Agodirin SO. The thyroid gland.medimag.com.ng vol1 level3
turn back upwards to rise above it and then descend in front of the bone. Again it turns
upwards rising behind the bone and then it finally turns downwards to descend to its
definitive position. This roaming or dancing motion forms an almost completely closed
“C” path around the body of the hyoid bone before it finally descends to its definitive
position. Some author believe the developing anlarge simply descend anterior or
posterior to the body of the hyoid bone or even through it .
The migrating thyroid gland/tissue is initially connected by a canalized stalk
(thyroglossal duct) to the floor of the mouth from where it arises. This duct should
normally disappears when the thyroid gland settles at its definitive position. If the stalk
connecting the fully developed gland to the floor of the mouth fails to disappear, it is
referred to as the thyroglossal duct. It tends to accumulate fluid and become clinically
obvious as thyroglossal cyst. During surgery for excision of the thyroglossal cyst or duct,
because the descending thyroid gland roams around the hyoid bone or descends through
the mid portion of the bone and core of the tongue should be excised along with the duct
and tract ( this is Sistrunk procedure) to be sure the whole remnant of the duct has been
excised. Failure to do this may be associated with recurrence rate as high as 85% or
formation of thyroglossal fistula, the later condition is never congenital.
The shape of the fully developed gland and surface anatomy
By the time the gland reaches its definitive position it is should have two lateral lobes
joined by the isthmus which is at the midline. The appearance of the adult organ (two
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Agodirin SO. The thyroid gland.medimag.com.ng vol1 level3
lateral lobes connected by a midline isthmus) has been described as shield shaped,
butterfly shaped or H –shaped. The gland is closely plastered to the voice box and upper
airway by the pretracheal fascia and its isthmus is attached to the cricoid cartilage by the
ligaments named after Berry as berry’s ligament (The recurrent laryngeal nerves ascend
in the tracheo-esophageal grove beside this ligament).
At its definitive position the isthmus lies opposite the 2nd to 4th tracheal cartilages. The
upper poles of the lateral lobes touch the oblique lines of the thyroid cartilage which is at
the level of the 5th cervical vertebra and the lower poles are at the level of the 7th cervical
vertebra. This is the surface anatomy of the normally situated gland. Any position
outside of this normal location is an ectopic location. The plastering by the pretracheal
fascia and attachment by the berry’s ligament will explain why the gland moves upwards
during swallowing. If we remember that during swallowing the voice box must be
protected from the food particles so it moves upwards to be closed by the epiglottitis
which descends to allow the swallowed food to roll over it into the oesophagus. Thus the
thyroid gland which is plastered to the voice box also moves up along with the voice box
as the patient swallows. This is the reason why patients with anterior or anterolateral
neck masses are asked to swallowing very early in the course of clinical examination of
their neck mass so that the clinician can immediately determine which path of
examination to progress with; whether to examine the mass as a thyroid mass or as any
other mass in the neck. The two patterns of examinations differ greatly.
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Each lateral lobe of the gland is conical in shape, as earlier noted, at the normal location
each lobe extends from the ipsilateral oblique line of the thyroid cartilage to the level of
the 5th tracheal ring or from the 5th to the 7th cervical vertebrae. The isthmus is opposite
the 2nd to the 4th tracheal cartilages. Each lateral lobe has dimension of 5 x 2x 3 (Height x
Width x Thickness. The height is at the longitudinal dimension, width is at the
transverse dimension and Thickness is antero-posterior dimension. ) These dimensions
bring to mind the dimensions of the normal prostate gland which is 4 x3 x 2 and the
testicle which is 5x3 x.2. The isthmus normally measures about 1.25cm in longitudinal
and transverse dimensions. The normal gland generally weighs between 15-30g.
Specifically it weighs about 0.3g per kg body weight. A gland that weighs up to 1g per kg
body weight is considered a giant goiter this is the objective definition of a giant goiter.
The subjective definition of giant goiter is when the gland is as big as the bearer’s head.
During the development of the thyroid gland, anomalies abound. This level 3 discussion
will continue with the congenital anomalies.
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