Thyroid gland

The thyroid gland is a single midline endocrine organ in the anterior neck responsible for thyroid hormone production. It lies in the visceral space, completely enveloped by the pretracheal fascia (the middle layer of the deep cervical fascia).

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 composed of two lobes, each with a superior and inferior pole. Usually, the superior pole is narrower than the inferior pole, giving each lateral lobe a pear-like shape. The lateral lobes are connected in the midline by a narrow isthmus adherent to the 2^nd to 4^th tracheal rings. 

Each lobe measures approximately 4 cm in length. The average weight is 25 g, slightly higher in females and may increase during menstruation and pregnancy ⁷.

The thyroid has a fibrous true capsule that extends as septae into the gland, forming lobes and lobules ^11,12. Surrounding this is a false capsule (a.k.a. thyroid gland sheath) derived from the pretracheal fascia. Between the true and false capsule is a space containing fat, nerves, vessels, and the parathyroid glands ^12,13. The false capsule thickens laterally to form the suspensory ligament of the thyroid ¹².

Berry's ligament is a posterior extension of the thyroid capsule that 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. It is an important surgical landmark during thyroidectomy 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), inferior thyroid artery

• posterolaterally: carotid space ³

Arterial supply

• superior thyroid artery (from the external carotid artery)

• 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 5^th 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

• pyramidal lobe: superiorly-projecting thyroid tissue from the isthmus

• thyroglossal duct cyst

• ectopic thyroid tissue

  • lingual thyroid

• accessory thyroid gland

• Zuckerkandl's tubercle

• 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 (sometimes called "perithyroid echogenic line" ¹⁴)

• each lobe normally measures ¹⁰

  • length or craniocaudal: 4-6 cm

  • depth or anterior-posterior dimension: <2 cm

  • isthmus <0.5 cm deep ⁴

• volume (excluding isthmus, unless its thickness is >3 mm) ⁶

  • 10-15 mL for females

  • 12-18 mL for males

CT

The thyroid is well-defined with homogeneous soft tissue density (60-80 HU) ¹².

MRI

The thyroid displays homogeneous signal, when compared to skeletal muscle that is T1 isointense and T2 hyperintense ¹².

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 a decrease in ¹³¹I uptake; eventually, however, the main phase is that of a hypothyroid state

• 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

• Hashimoto thyroiditis

  • F>M 10:1

  • associated with

    • Down syndrome and Turner syndrome

    • autoimmune conditions including systemic lupus erythematosus, Sjögren syndrome and myasthenia gravis

    • primary thyroid lymphoma(B cell)

  • 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 to adjacent tissues

  • typically presents as a hard goiter which commonly compresses the trachea; it is hypoenhancing and hypoechogenic

  • associated with: 

    • retroperitoneal fibrosis

    • primary sclerosing cholangitis (PSC)

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

• cracking thyroid

• shock thyroid

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.