The thyroid gland is a single midline endocrine organ in the anterior neck responsible for thyroid hormone production which lies in the visceral space completely enveloped by pretracheal fascia (middle layer of the deep cervical fascia).
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Gross anatomy
The thyroid extends from C5 to T1 and lies anterior to the thyroid and cricoid cartilages of the larynx and the first five or six tracheal rings.
The thyroid is butterfly or "H"-shaped and is composed of two lobes, each with a superior and inferior pole. Usually, the superior pole is narrower than the inferior pole giving a pear-like shape to each lateral lobe. The lateral lobes are connected in the midline by a narrow isthmus which is adherent to the 2nd to 4th tracheal rings. Each lobe measures approximately 4 cm in length. The average weight is 25 g; this is slightly higher in females and may increase during menstruation and pregnancy 7.
The parathyroid glands lie posteromedially and are sometimes intracapsular.
The ligament of Berry is a posterior extension of the thyroid capsule which attaches to the cricoid cartilage and the upper tracheal rings. It encloses a short segment of the recurrent laryngeal nerve as it ascends in the tracheo-esophageal groove. As such it is an important surgical landmark during thyroidectomies to avoid damaging the nerve.
Relations
anteriorly: strap muscles
posteriorly: thyroid cartilage, cricoid cartilage, trachea
posteromedially: tracheo-esophageal groove (containing lymph nodes, recurrent laryngeal nerve, parathyroid glands)
posterolaterally: carotid space 3
Arterial supply
superior thyroid artery (from the external carotid artery)
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inferior thyroid artery (from the thyrocervical trunk)
if the inferior thyroid artery arises from the subclavian artery it is referred to as an accessory inferior thyroid artery
Venous drainage
superior thyroid vein (drains to the internal jugular vein)
middle thyroid vein (drains to the internal jugular vein)
inferior thyroid vein (drains via plexus to the brachiocephalic vein)
Lymphatic drainage
Lymphatic drainage is multidirectional and initial lymph drainage is to perithyroid lymph nodes then on to prelaryngeal, pretracheal and paratracheal nodes (level 6 lymph nodes).
Innervation
Sympathetic supply is provided by superior, middle, and inferior cervical ganglia.
Embryology
The thyroid gland develops from the proximal primitive foregut between the first and second pharyngeal pouches at the foramen cecum, in the midline of the base of the tongue. During the 5th embryonic week, a diverticulum forms at the foramen which inferiorly migrates anterior to the body of the hyoid bone, curving posterior and superiorly to reach behind the bone before once more turning inferiorly and continuing anterior to the larynx, forming the thyroglossal duct 1,3,7. The tip of the duct bifurcates, forming the two lobes of the gland. The parafollicular cells (C cells) responsible for calcitonin production are derived from separate tissue, the ultimobranchial body, a small diverticulum of the fourth pharyngeal pouch 7.
Variant anatomy
lobar hemiagenesis
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superiorly-projecting thyroid tissue from the isthmus
the gland may be supplied by a thyroidea ima artery, which may replace the inferior thyroid artery (3%)
Radiographic appearance
Ultrasound
the normal thyroid gland has a homogeneous appearance with medium echogenicity 9
the capsule may appear as a thin hyperechoic line
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each lobe normally measures 4
length: 4-7 cm
depth: <2 cm
isthmus <0.5 cm deep
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volume (excluding isthmus, unless its thickness is >3 mm) 6
10-15 mL for females
12-18 mL for males
Related pathology
Neoplasms
Inflammatory conditions: thyroiditis
Autoimmune
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de Quervain thyroiditis (subacute granulomatous)
postviral
associated with HLA-B35
acutely there is an increase in thyroid hormone, with a resultant decrease in TSH; as a result, there is a decrease in 131I uptake; eventually, however, the main phase is that of a hypothyroid state
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subacute lymphocytic thyroiditis
painless
young women, especially in the postpartum period
gland is usually normal in size, or minimally increased
usually an early hyperthyroid state which returns to normal, but may have a transient late hypothyroid period
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F>M 10:1
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associated with
autoimmune conditions including systemic lupus erythematosus, Sjögren syndrome and myasthenia gravis
anti-TSH-receptor-autoantibodies
CD8+ cytotoxic T-cell mediated
usually hypothyroid, although there may be a brief hyperthyroid early phase
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distinctive in that inflammation extends beyond the confines of the gland to adjacent tissues
typically presents as a hard goiter which commonly compresses the trachea; it is hypoenhancing and hypoechogenic
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associated with:
NB: although Graves disease is autoimmune it is not really a thyroiditis as it does not have a significant inflammatory component.
Infective
Includes thyroiditis associated with:
pneumocystis jiroveci infection (PCP/PJP)
mycobacterium avium complex (MAC)
fungal
Others
History and etymology
"Thyroid" derives from the Greek word "θνρεός" (large oblong shield) and "είδος" (-like) 5. The Greek shield had a notch cut into it for the chin, and the resemblance of the shield to a particular piece of cartilage in the neck brought about the term "thyreoid cartilage" (the "e" was later dropped).
The thyroid gland was originally known as the "laryngeal gland", and was renamed the "thyroid gland" by Thomas Wharton in 1646.
The word isthmus is derived from the Ancient Greek ισθμος (isthmos) meaning a neck of land 8.
