Thyroid

Thyroid

  • Located anteroinferior neck at level of thyroid cartilage

  • Straddles trachea anteriorly

  • Bounded laterally by carotid arteries and jugular veins

  • Pyramidal lobe arise from isthmus

  • Size:

    • Lobes equal in size

    • Normal adult thyroid: 40-60 L x 20 W x 13-18 H mm

    • Isthmus: 4-6 mm AP diameter

  • Volume:

    • Normal mean thyroid volume is 18.6 +/- 4.5 (14.1-23.1) ml

    • Volume in males is slightly larger than in females

  • Relational anatomy:

    • Anterior

      • Anterior surface: strap muscles, including sternothyroid, omohyoid, sternohyoid, and sternocleidomastoid muscles

      • Sternohyoid and omohyoid muscles: thin, hypoechoic bands anterior to gland

      • Sternocleidomastoid muscles: large oval band that lies anterior and lateral to gland

    • Posterior

      • Posterolateral anatomy: Common carotid artery, internal jugular vein, vagus nerve

      • Longus colli muscle: posterior and lateral to lobes. Hypoechoic triangular structure adjacent to cervical vertebrae

    • Medial

      • Medial anatomy: larynx, trachea, inferior constrictor of pharynx, esophagus

      • Esophagus, primarily a midline structure, may be found to the left od trachea

      • Identified by target appearance in transverse plane and by its peristaltic movement

  • Blood Supply

    • Two superior thyroid arteries arise from external carotids and descend to upper poles

    • Two inferior thyroid arteries arise from thyrocervical trunk of subclavian artery and ascend to lower poles

    • Corresponding veins drain into internal jugular veins

  • Physiology and lab values

    • Maintains normal body metabolism, growth, and development by synthesis, storage, and secretion of thyroid hormones

    • Mechanism for producing thyroid hormones is iodine metabolism

    • Thyroid gland traps iodine from blood and through series of chemical reactions: produces thyroid hormones T3 and T4

    • When thyroid hormone needed by body, it is released into bloodstream by action of thyrotropin, or thyroid-stimulating hormone (TSH), produced by pituitary gland

    • Calcitonin decreases concentration of calcium in blood by first acting on bone to inhibit its breakdown

    • Calcitonin helps to maintain homeostasis of blood calcium

    • *Calcitonin is less common

    • When thyroid is producing correct amount of thyroid hormone, considered to be normal , euthyroid

  • Tests of thyroid function

    • Nuclear medicine used to determine function of thyroid: iodine uptake scan and thyroid scan

    • Lab tests for thyroid measure amount of T3 or T4 in blood

      • Amounts elevated in patients with hyperthyroidism and decreased in patients with hypothyroidism

  • Sonographic evaluation of the thyroid

    • Patient placed in supine position with pillow under both shoulders to provide moderate hyperextension of neck

    • High frequency (7.5-15 MHz) linear-array transducer should be used

    • Elastography: demonstrate soft areas as blue and hard eras as red within the thyroid gland

       

Parathyroid

  • Parathyroid glands normally located on posterior medial surface of thyroid gland

  • Four parathyroid glands (some have three or five)

  • Have been found in ectopic places, such as in neck and mediastinum

  • Paired. Two lie posterior to each superior pole of thyroid; two lie posterior to inferior pole

  • Each gland flat and disc-shaped

  • Echo texture similar to that of overlying thyroid gland

  • Normal-size glands (<4mm) usually not seen with sonography

  • Enlarged glands (>5mm) have decreased echo texture and appear sonographically as elongated masses between posterior longus colli and anterior thyroid lobe

  • Physiology and Lab data

    • Calcium-sensing organs in body

    • Produce parathyroid hormone (PTH) and monitor serum calcium feedback mechanism

    • Stimulus to PTH secretion is decrease in level of blood calcium

    • When serum calcium level decreased, parathyroid glands are stimulated to release PTH

    • Serum calcium level increases; parathyroid activity decreases

    • PTH acts on bone, kidney, and intestine to enhance calcium absorption

    • Unexplained hypercalcemia detected on routine blood chemistry screening is most common referrals for parathyroid sonography

    • Symptomatic renal stones, ulcers, and bone pain are other indications

  • Sonographic evaluation of the parathyroid gland

    • High-resolution (7.5-15 MHz) linear transducer

    • Patient placed supine with neck slightly hyperextended

    • Upper neck (jaw to sternal notch), transverse and longitudinal planes of thyroid/parathyroid area

    • Patient asked to swallow to elevate

    • Size:

      • 5 mm x 3 mm x 1 mm

      • Oval or bean-shaped. Also reported as spherical, elongated, or lobulated

    • Longus colli muscle: appears as discrete area posterior to thyroid

    • Longitudinal sections: linear appearance of muscle evident in this plane

    • Minor neurovascular bundle may also be source of confusion

    • Longitudinal scans can often eliminate confusion by identifying bundle's tubular appearance