Thyroid and Parathyroid Glands

GROSS ANATOMY

Thyroid Gland

  • 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

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

    • Posteromedially are tracheoesophageal grooves

      • Contains paratracheal lymph nodes, recurrent laryngeal nerve, parathyroid glands

    • Posterolaterally are carotid spaces

      • Contains common carotid artery, internal jugular vein, vagus nerve

    • Anteriorly are infrahyoid strap muscles

    • Anterolaterally are sternocleidomastoid muscles

  • Thyroid gland

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

      • Measure ~ 4 cm in height

      • Each lobe has upper and lower poles

      • Lateral lobes are commonly asymmetric in size

    • Lateral lobes are joined by midline isthmus

    • Pyramidal lobe present in 30-50% of cases

      • Extends superiorly from isthmus toward hyoid bone

      • More common on left

  • Superior thyroid arteries

    • 1st anterior branch of external carotid artery

  • Inferior thyroid arteries

    • Arise from thyrocervical trunk, branch of subclavian artery

  • Thyroidea ima occasionally present (3%)

    • Single vessel originating from aortic arch or brachiocephalic artery

    • Enters thyroid gland at inferior border of isthmus

  • Lymphatic drainage

    • Lymphatic drainage is extensive and multidirectional

    • Initial lymphatic drainage courses to periglandular nodes

      • Prelaryngeal, pretracheal, and paratracheal nodes along recurrent laryngeal nerve

      • Paratracheal nodes drain into mediastinum

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

  • Embryology

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

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

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

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

Parathyroid Glands

  • Small lentiform glands posterior to thyroid gland

    • Extracapsular in most cases but can be located within thyroid gland

  • Located in region of tracheoesophageal groove

  • Normal measurements

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

  • Variable number, but typically 4

    • 2 superior and 2 inferior

  • Superior parathyroid glands

    • More constant in position as compared with lower glands

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

  • Inferior parathyroid glands

    • More variable in location

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

    • 15% lie within 1 cm of inferior thyroid poles

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

ANATOMY IMAGING ISSUES

Thyroid Gland

  • 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

    • 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)

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

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

    • Shape : Wider than tall, taller than wide

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

    • 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

Parathyroid Glands

  • Normal glands often not seen

    • Look carefully in tracheoesophageal groove behind mid- to lower thyroid

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

  • Best 1st examination for localizing parathyroid adenoma

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

    • Hypoechoic on grayscale with hypervascularity on color flow imaging

      • Color flow imaging best done in longitudinal plane

  • Easy to confuse with other structures, especially cervical lymph nodes

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

THYROID AND PARATHYROID GLANDS

Nov 10, 2024 | Posted by in ULTRASONOGRAPHY | Comments Off on Thyroid and Parathyroid Glands

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