H- or U-shaped gland in anterior cervical neck formed from 2 elongated lateral lobes with superior and inferior poles connected by median isthmus

Thyroid gland lies anterior and lateral to trachea in visceral space of infrahyoid neck

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  Extends from level of 5th cervical vertebra to 1st thoracic vertebra

  
  
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  Posteromedially are tracheoesophageal grooves

  
  
  
  
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    Contains paratracheal lymph nodes, recurrent laryngeal nerve, parathyroid glands

  
  
  
  
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  Posterolaterally are carotid spaces

  
  
  
  
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    Contains common carotid artery, internal jugular vein, vagus nerve

  
  
  
  
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  Anteriorly are infrahyoid strap muscles

  
  
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  Anterolaterally are sternocleidomastoid muscles

Thyroid gland

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  2 lateral lobes (i.e., right and left lobes)

  
  
  
  
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    Measure ~ 4 cm in height

    
    
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    Each lobe has upper and lower poles

    
    
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    Lateral lobes are commonly asymmetric in size

  
  
  
  
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  Lateral lobes are joined by midline isthmus

  
  
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  Pyramidal lobe present in 30-50% of cases

  
  
  
  
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    Extends superiorly from isthmus toward hyoid bone

    
    
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    More common on left

  
  

Superior thyroid arteries

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  1st anterior branch of external carotid artery

Inferior thyroid arteries

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  Arise from thyrocervical trunk, branch of subclavian artery

Thyroidea ima occasionally present (3%)

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  Single vessel originating from aortic arch or brachiocephalic artery

  
  
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  Enters thyroid gland at inferior border of isthmus

Lymphatic drainage

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  Lymphatic drainage is extensive and multidirectional

  
  
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  Initial lymphatic drainage courses to periglandular nodes

  
  
  
  
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    Prelaryngeal, pretracheal, and paratracheal nodes along recurrent laryngeal nerve

    
    
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    Paratracheal nodes drain into mediastinum

    
    
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    Regional drainage occurs laterally into internal jugular chain (levels 2-4) and spinal accessory chain (level 5)

  
  

Embryology

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  Thyroid gland forms near base of tongue (foramen cecum) and descends into neck along thyroglossal duct

  
  
  
  
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    Ectopic thyroid gland can be seen from base of tongue into superior mediastinum

    
    
    
    
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      Most common location is just deep to foramen cecum at tongue base (i.e., lingual thyroid)

    
    
    
    
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    Thyroglossal duct cyst results from failure of complete involution of thyroglossal duct

  
  

Small lentiform glands posterior to thyroid gland

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  Extracapsular in most cases but can be located within thyroid gland

Located in region of tracheoesophageal groove

Normal measurements

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  ~ 6 mm in length, 3- to 4-mm transverse, and 1-2 mm in anteroposterior diameter

Variable number, but typically 4

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  2 superior and 2 inferior

Superior parathyroid glands

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  More constant in position as compared with lower glands

  
  
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  Lie on posterior border of middle 1/3 of thyroid in 75%

Inferior parathyroid glands

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  More variable in location

  
  
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  50% of inferior glands lie lateral to lower pole of thyroid gland

  
  
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  15% lie within 1 cm of inferior thyroid poles

  
  
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  35% position is variable, residing anywhere from angle of mandible to lower anterior mediastinum

Use high-resolution linear array transducer (10-15 MHz)

Normal thyroid parenchyma has fine, uniform echoes and is hyperechoic compared to adjacent muscles

Echogenic thyroid capsule is clearly visualized and helps to differentiate thyroid lesions from extrathyroidal masses

Both longitudinal and transverse scans are required for comprehensive ultrasound assessment of thyroid gland

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  Measurements taken in all 3 planes

Midline transverse view for isthmus

Document size of all nodules with particular attention to sonographic characteristics (all features are listed from least concerning to most concerning for malignancy)

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  Composition : Cystic, spongiform, mixed cystic and solid, solid, or predominantly solid

  
  
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  Echogenicity : Anechoic, hyperechoic, isoechoic, hypoechoic, very hypoechoic

  
  
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  Shape : Wider than tall, taller than wide

  
  
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  Margin : Smooth, ill-defined, lobulated or irregular, extrathyroid extension

  
  
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  Echogenic foci : Comet-tail artifacts, macrocalcifications, peripheral (rim) calcifications, punctate echogenic foci

Assess adjacent structures (including trachea, esophagus, strap muscles, carotid artery, and internal jugular vein) and cervical lymph nodes

Normal glands often not seen

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  Look carefully in tracheoesophageal groove behind mid- to lower thyroid

  
  
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  Small, well-circumscribed, hypoechoic nodules posterior to thyroid gland separated by echogenic thyroid capsule

Best 1st examination for localizing parathyroid adenoma

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  May be in ectopic location so start above thyroid at angle of mandible and move downward, scanning through thyroid to level of clavicle

  
  
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  Hypoechoic on grayscale with hypervascularity on color flow imaging

  
  
  
  
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    Color flow imaging best done in longitudinal plane

  
  

Easy to confuse with other structures, especially cervical lymph nodes

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  Look for echogenic hilum and classic central hilar vessel in lymph node