Thyroid gland

The thyroid gland is an endocrine organ in the neck which is completely enveloped by the middle-layer of the deep cervical fascia and lies in the visceral space. 

Gross anatomy

The thyroid extends from C5 to T1 and lies anterior to the thyroid and cricoid cartilages of the larynx and the first three tracheal rings. 

The thyroid is butterfly or "H"-shaped and is composed of two lobes, each with a superior and inferior pole, connected by an isthmus. Each lobe measures approximately 4 cm in length.

The parathyroid glands lie posteromedially and are sometimes intracapsular. 

Relations

• anteriorly: strap muscles
• posteriorly: thyroid cartilage, cricoid cartilage, trachea
• posteromedially: tracheo-oesophageal groove (containing lymph nodes, recurrent laryngeal nerve, parathyroid glands)
• posterolaterally: carotid space ³

Blood supply

• arterial supply
  • superior thyroid artery (from external carotid artery)
  • inferior thyroid artery (from 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 internal jugular vein)
  • middle thyroid vein (drains to internal jugular vein)
  • inferior thyroid vein (drains to brachiocephalic vein)

Lymphatic drainage

Lymphatic drainage is multi-directional and initial lymph drainage is to peri-thyroid lymph nodes then onto prelaryngeal, pretracheal and paratracheal nodes (level 6 lymph nodes). 

Embryology

The thyroid develops from the 1^st and 2^nd pharyngeal pouches at the foramen caecum in the midline in the region of tongue in the embryonic period and descends to its final position ^{1, 3}.

Variant anatomy

• lobar hemiagenesis
• pyramidal lobe
  • superiorly-projecting thyroid tissue from the isthmus
• thyroglossal duct cyst
• ectopic thyroid tissue
  • lingual thyroid
• Zuckerkandl's tubercle
• may be supplied by a thyroidea ima artery, which may replace the inferior thyroid artery (3%)

Radiographic appearance

Ultrasound

• normal thyroid gland has an homogenous appearance, the capsule may appear as a thin hyperechoic line
• each lobe normally measures ⁴
  • length: 4-7 cm
  • depth: <2 cm
  • isthmus measures <0.5 cm deep 
• volume (excluidng isthmus, unless its thickness is > 3mm )⁶
  • 10-15 ml for females
  • 12-18 ml for males

Related pathology

Neoplasms

• assessment of thyroid lesions
• thyroid neoplasms
• thyroid cancer staging

Inflammatory conditions: thyroiditis

Autoimmune

• Graves disease
• 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 decrease in I131 uptake; eventually however, the main phase is that of a hypothyroid state
• subacute: lymphocytic thyroiditis
  • painless
  • young women, especially in 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
• Hashimoto thyroiditis
  • F>M 10:1
  • associated with Down syndrome and Turner syndrome
  • SLE, Sjögren, myasthenia gravis, B-cell NHL
  • anti-TSH-receptor-autoantibodies
  • CD8+ cytotoxic T cell mediated
  • usually hypothyroid, although there may be a brief hyperthyroid early phase
• Riedel thyroiditis
  • distinctive in that inflammation extends beyond the confines of the gland in to adjacent tissues
  • typically presents as a hard goitre which commonly compresses the trachea; it is hypoenhancing and hypoechogenic
  • associated with: 
    • retroperitoneal fibrosis
    • primary sclerosing cholangitis (PSC)

Note: 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:

• PCP/PJP
• MAIC
• fungal

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.