Thyroid gland

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

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. Average weight is 25g.

The parathyroid glands lie posteromedially and are sometimes intracapsular. 

Relations

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). 

Sympathetic supply provided by superior, middle and inferior cervical ganglia.

The thyroid develops from the 1st and 2nd 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.

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 (excluding isthmus, unless its thickness is > 3mm) 6
    • 10-15 ml for females
    • 12-18 ml for males
Neoplasms
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

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

"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.


Head and neck anatomy
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Article Information

rID: 2178
System: Head & Neck
Section: Anatomy
Synonyms or Alternate Spellings:
  • Thyroid glands
  • Thyroid

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Cases and Figures

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    Figure 1: FNA - subacute (de Quervain's) thyroiditis
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    Case 1: left thyroid lobe agenesis
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    Lingual thyroid
    Case 2: lingual thyroid
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    Case 3: thyroidea ima artery
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