THYROID/PARATHYROID

I. Introduction/General Information

A. Thyroid

1. Endocrine gland

a. Lobes are cone shaped

b. Apex extends to oblique line of thyroid cartilage

2. Highly vascular

3. Anterior & lateral to larynx, trachea4. Lobes connected by

an isthmus5. Pyramidal lobe may

be present6. Normally not

palpable

Thyroid, General Information, continued …

The Thyroid Gland

Located in the anterior region of the neck

This gland has an accessory (pyramidal) lobe

Thyroid Gland, Anterior and Posterior Views

Thyroid Gland: anterior view (left); and posterior view (right)

Thyroid, General Information, continued …

8. Isthmus crosses tracheal cartilages 2-4

9. Base located ~4-5th tracheal cartilage

10. Thyroxin function: regulates basic metabolism in all cells

Thyroid, General Information, continued

B. Parathyroid glands

1. Usually four – two on each side (2-8 is normal)

2. Lie on the posterior surface of thyroid

3. May be embedded within thyroid gland

4. Regulate calcium/phosphate levels5. Required for life

Parathyroid Glands (Post. view of thyroid)

Parathyroid Glands are located on the posterior aspect of the thyroid; sometimes the tissue is embedded within thyroid tissue.

II. Detailed Anatomy

A. Thyroid

1. Largest endocrine gland in adult

2. Normally extends from ~ C-5 through ~ T-1

3. Highly vascular

4. Weights ~20 - 30 grams

Thyroid, Detailed Anatomy, continued …

5. Pyramidal lobe

a. present in ~ 33% of population.

b. Extends upward from isthmus

c. anterior to thyroid cartilage

Thyroid, Detailed Anatomy, continued …

d. Embryologic remnant of thyroglossal duct

e. formerly connected cecum of tongue to thyroid diverticulum

f. Site of developing gland

Pyramidal Lobe of the Thyroid Gland

Pyramidal Lobe

Thyroid Gland, Anterior View

Thyroid, Detailed Anatomy, continued …

c. Lobes:

1. Attached to cricoid cartilage by ligaments

2. Medial surface adapted to larynx and trachea

Thyroid, Detailed Anatomy, cont …

3. Lobes related posteriorly to the esophagus

4. Posterolateral surface

a. related to carotid sheath

b. overlaps carotid artery

Thyroid, Detailed Anatomy, con’t…

d. Isthmus

1. 1.25 cm x 1.25 cm

2. Crosses @ tracheal rings ~2-4

3. Occasionally absent

Thyroid gland vascular, continued …

6. Highly vascular gland supplied by four large arteriesa. R & L inferior thyroid arteryb. R & L superior thyroid artery

Thyroid gland vascular, continued …

7. Drained by R & L superior, middle and inferior thyroid veins

a. Veins arise from plexus b. on anterior surface of gland

c. Extend over anterior surface of trachea

Inferior Thyroid

Vein

Thyroid gland, continued …

d. Lymph vessels 1. In interlobular connective

tissue between lobes 2. Connect with network in wall of gland 3. terminate in thoracic and right lymphatic ducts

Thyroid gland, continued …

8. Muscular landmarks a. Sternocleidomastoid muscles lie laterallyb. Longus colli (prevertebral) muscles lie posteriorlyc. “Strap” muscles lie anteriorly

Strap Muscles

Thyroid gland, continued …

9. Autonomic innervation via

a. Cervical portion of sympathetic trunk

b. Parasympathetic fibers arise from Vagus X

Detailed Anatomy, continued …

9. Treatment considerations

a. Superior thyroid artery is accompanied by superior

laryngeal nerve

b. Inferior thyroid artery is accompanied by recurrent

laryngeal nerve

Detailed Anatomy, continued …

c. Damaging nerves results in partial or total paralysis of larynx

d. Bordered by neurovascular structures contained in carotid sheath

Thyroid gland, continued …

11. Pretracheal fascia: a. surrounds thyroidb. Extends from hyoid bone to

fibrous pericardiumc. Encloses trachea, larynx,

pharynx, thyroidd. Parathyroids and venous

plexus lie between layers

Pretracheal fascia, continued …

d. Fascia more dense anteriorly1. Thyroid enlargement often occurs posteriorly2. May compress trachea,

esophagus3. 1st symptom may be hoarseness, difficulty swallowing

Parathyroid Gland, Detailed Anatomy

B. Parathyroid glands

1. Yellow-brown

2. ovoid or lentiform structures

3. weigh ~ 50 mg each

4. Measure 3-10 mm x 2-6 mm x 1-4 mm

Parathyroid Glands, con’t….

5. Lie between posteromedial thyroid lobes and carotid sheath

6. Close proximity to:

a. Tracheoesophageal groove

b. longus colli muscles

Parathyroid Glands, continued …

7. Position of superior glands is more predictable

8. Aberrant glands may lie between trachea and thyroid

9. Blood, lymphatic and nerve supply: same as for thyroid

III. Thyroid/Parathyroid Diseases

A. Ultrasound useful in differentiating cystic from complex or solid

massesB. Thyroid Cysts

1. Typical cystic appearance2. Colloid cysts3. Thyroglossal duct cysts

Pathology, cont…

3. Branchial cleft cysts

a. More specific than pattern for other

neoplasms

b. Are usually more cephalic

4. Only 20% of thyroid masses are simple cysts

Pathology, cont…

C. Structures of Carotid Sheath1. Jugular vein, carotid artery2. Delineate lateral aspect of thyroid3. Jugular vein lateral to carotid artery

a. vein has greater diameter b. Is distensible on valsalva

maneuver

Transverse Section of Thyroid Gland

Internal Jugular Vein

Thyroid Gland, Left Lobe

Trachea

Esophagus

Common Carotid Artery

Sternocleidomastoid Muscle

Brachial Plexus

Gray-Scale anatomy, continued

D. Thyroid is more homogeneous and echogenic than surrounding muscle1. Sternocleidomastoid (lateral)2. Longus colli (posterior)

E. US is less helpful w/ complex masses or diffuse parenchymal disorders

Gray-Scale Anatomy, continued …

F. Parathyroids are difficult to see

1. Size and location are variable

2. Usually:

a. moderately echogenic b. well-circumscribed

c. capsule around d. anterior to longus colli e. medial to common carotid a.

Parathyroids, continued …

3. Longus Colli (“prevertebral”) Musclesa. Attachments:

1. O = cervical vertebrae2. I = cervical vertebrae

b. Action: twists, bends neckc. Lie posterior

IV. Thyroid Diseases

A. Metastasis from thyroid cancer

1. May invade local structuresa. trachea b. esophagus c. carotid artery d. jugular vein

Thyroid Diseases, con’t…

2. Innervation may be involved

a. Voice, speech changes

b. Horner’s Syndrome of eye1. droopy eye2. dryness3. small pupil

Thyroid Diseases, continued …

B. Cystic Masses

1. Colloid cyst:

a. Contains thyroglobulin

b. May have hemorrhagic center

c. May be aspirated

Thyroid Follicles showing Colloid

Thyroid Follicles with colloid containing Thyroid hormones

Thyroid Diseases, continued …

2. Branchial cleft cyst

a. Usually more cephalic

b. Non-closure of a branchial cleft

c. lie laterally

Thyroid Diseases, continued …

3. Thyroglossal Duct Cyst

a. Lies @ midline

b. Represents non-closure of thryoglossal duct

c. Congenital anomaly:

1. retention of tract

2. between thyroid and foramen cecum

Thyroglossal Duct Cyst, Thyroid Disease, continued …

d. Usually surrounded by hyoid bone

e. More frequently diagnosed in pediatric age groups

f. may be asymptomatic

Thyroid, continued …

g. Incidence in patients undergoing thyroid surgery:

1. 4% have this cyst

2. 28% of those with the cyst are > 50 years old

h. Symptoms: painless swelling at midline of neck

Thyroid, continued …

i. Treatment:

1. excision of cyst & central hyoid bone

2. < 1% accompanied by cancer

j. Account for ~ 70% of congenital cysts of neck

k. may appear at any location along duct

Thyroid Diseases, continued …

C. Complex/Solid Masses (Neoplasm)

1. Adenoma

a. Well-encapsulated

b. Usually solitary

c. Homogeneous

Thyroid Diseases, Adenomas, continued …

c. May be complex, with

1. sonolucent halo 2. echogenic

center

d. 50% of thyroid cancers are papillary adenocarcinomas

2. Complex solid masses show increased malignancy

Adenoma of the Thyroid Gland

Note atrophy of the left lobe with the tumor protruding from it

Right lobe may hypertrohy to compensate for loss of function in L. lobe

Thyroid, continued …

3. Goiter

a. Enlargement of thyroid gland

b. due to insufficient iodine

c. Gland appears nodular with irregular outline

d. Grossly enlarged

Goiter, continued …

e. Nodules have variable echogenicity

f. Treatable with iodine in diet

g. More common inland, and before iodized salt

Types of Goiters

Simple Goiter (L) and Nodular (Toxic) Goiter (R)

D. Parathyroid Diseases

1. Benign adenoma

a. Relatively common

b. usually results in

hyperparathyroidism

2. Cancers are rare

3. Surgical excision gives > 90% cure rate

Parathyroid Diseases

Pathogenesis of Hyperparathyroidism PTH increases blood

calcium levels Acts on bone,

kidneys, small intestines

Long-term Effects on the Skeletal System

Hyperparathyroidism