ANATOMY OF THYROID GLAND

AHAMMED KABEER M A55TH BATCH GOV MED COLLEGE CALICUT

INTRODUCTIONDEVELOPMENTGROSS ANATOMYBLOOD SUPPLYNERVE SUPPLYLYMPHATIC DRAINAGEHISTOLOGYSURFACE ANATOMYAPPLIED ANATOMY

• Largest endocrine gland• Thyroid hormones (T3 , T4 )- BMR• Thyrocalcitonin – antagonize PTH (i.e decrease serum Ca)• Highly vascular*

DEVELOPMENT OF THYROID GLAND

• Median endodermal thyroid diverticulum• Thyroglossal duct• Tuberculum impar• Foramen caecum• Lateral thyroid (4th pouch n ultimobranchial body) – parafollicular cells

SITUATION

C5

T1 / 4th or 5th tracheal ring

2nd

4th

Tracheal rings

DIMENSIONS AND WEIGHT

• Butterfly/H shaped• Lobes 5*3*2cm• Isthmus 1.2*1.2cm• Weight 25g• Larger in females • Enlarges in pregnancy & menstrution

CAPSULES

• True capsule (fibrous) – condensation of CT of gland• Septae & lobules• Arteries and plexus of veins deep to it• False capsule – pretracheal fascia• Moves during deglutition and speech• Suspensory ligament of Berry*

Deep cervical fascia

RELATIONSs

1. Lobes • Conical• Apex, base • 3 surfaces – L, M, PL• 2 borders – A, P• Apex – oblique line of thyroid cartilage*• Base – 4th/5th tracheal ring

• Apex is sandwitched b/w inf constrictor and sternothyroid I.e upward extension is restricted

• Lateral surface – convex, • Covered with sternohyoid, SCM, superior belly of omohyoid,

sternothyroid

Deep cervical fascia

• Medial surface- 2 tubes(larynx-trachea) and (pharynx-oesophagus) - 2 muscles cricothyroid, inferior constrictor - 2 nerves external and recurrent laryngeal N• PL surface – carotid sheath

• Ant border – anterior branch of STA

• P border - ITA - anastomosis b/w STA & ITA - parathyroid glands - thoracic duct on left

2. Isthmus• 2 surfaces A & P• 2 borders Sup & Inf• A surface – skin & fascia - anterior jugular veins - R & L sternohyoid & sternothyroid• P surface- 2nd – 4th tracheal rings• Sup border – anastomosis b/w R&L STA• Inf border - ITV leave

Pyramidal lobe

• Extend superiorly from isthmus/left lobe• Attached to body of hyoid bone by fibromuscular band• Levator glandulae thyroidae( LGT)• U/L or B/L• LGT represent detached part of infrahyoid muscles• May be innervated by ansa cervicalis• Remnant of thyroglossal duct

BLOOD SUPPLY – Arterial supply

1. Superior thyroid artery (STA)• 1st ant br of ECA• External laryngeal N *• Ligature *• Pierce pretracheal fascia at apex• Divide into A & P branches• A branch- ?• P branch?

2. Inferior thyroid artery (ITA)• Largest branch of thyrocervical trunk (SCA)• Pass behind carotid sheath, MCG and in front of vertebral A• Terminate near lower pole• Recurrent laryngeal N*• Ligation*• Ascending br*?• Glandular br?

3. Thyroidea ima artery*( lowest thyroid artery) 3%• from arch of aorta/brachiocephalic trunk/right common carotid/right

subclavian/ internal thoracic A• Enter lower border of isthmus• Tracheostomy*4. Accessory thyroid arteries

VENOUS DRAINAGE1. Superior thyroid vein (STV)

• Accompany STA

• Drain to IJV/ facial v

2. Middle thyroid vein

• Very short

• From middle of lobe

• Drain to IJV

3.Inferior thyroid vein*• Plexus on trachea• Drain to left brachiocephalic vein• R – passes ant to innominate a R BCV or ant trachea L BCV• L – drainage L BCV• **occ – both inf veins form a common trunk “thyroid ima vein” empties into L BCV • Thyroid vein of Kocher* emerge from lower pole drain to IJV profuse bleeding4. kocher’s thyroid vein*

LYMPHATIC DRAINAGE

• Extensive, multidirectional flow• periglandular prelaryngeal (Delphian) pretracheal paratracheal

(along RLN) brachiocephalic (sup mediastinum) deep cervical thoracic duct• Upper part via prelaryngeal LN to upper deep CLN• Lower part via pretracheal and paratracheal LN to lower deep CLN• Brachiocephalic LN and thoracic duct• regional metastasis of thyroid carcinoma are superior and lateral, along

IJV ie: invasion of the pretracheal and paratracheal LNs and obstruction of normal lymph flow

INNERVATION

• Vasoconstrictor sympathetic innervation• Mainly from MCG• Partly from SCG and ICG• Cardiac and laryngeal branches of vagus(parasympathetic)• Enter along wuth blood vessels• Never secretomotor (secretion regulated by TSH)

HISTOLOGY

• Septae from fibrous capsule• Lobules• Follicles filled with colloid• Follicular cells- T3 , T4 ( level of activity)• Parafollicular cells( Clear cells, pale cells)- thyrocalcitonin

(ultimobranchial body), an APUD cell• Colloid – iodothyroglobulin

SURFACE ANATOMY

• Anterior triangles in the lower neck on either side of the air way and digestive tract inferior to the position of the oblique line of the thyroid cartilage .• sternothyroid muscles- oblique line of thyroid cartilage ,prevent the

lobes from moving upwards • Palpated by finding the thyroid prominence and arch of the cricoid

cartilage and then feeling posterolateral to the larynx. • Isthmus crosses anterior to the upper end of the trachea and can be

easily palpated in the midline inferior to the arch of the cricoid. • Presence of isthmus makes palpating the tracheal cartilages difficult and difficult tracheostomy.

• Isthmus marked by –

• Lobes extend – apex to middle of thyroid crtilage - base to clavicle - laterally overlapped by ant border of SCM

Arch of cricoid

1.2cm

1.2cm

1.2cm

APPLIED ANATOMY

• Presence of thyroidae ima A- chance of profuse bleeding procedures in neck below isthmus• Thyroglossal duct cysts – remnants of thyroglossal ducts at any point in the way of descent,(midline near hyoid)• Pyramidal lobe and presence of levator glandulae thyroidae• Ectopic thyroid glands – lingual/higher placed• Accessory thyroid glands – in thymus/ on thyrohyoid muscle• Non neoplastic, noninflammatory enlargement – goiter

• pressure symptoms and nerve involvments are common in goiter and carcinoma• Compression of trachea, carotid sheath, and venous engorgement

• Endemic goiter - I2 deficient soil/water• Subtotal thyroidectomy often preferred in hyperthyroidism – -to preserve external and recurrent laryngeal N - to spare parathyroid glands - to prevent post operative myxoedema• Injury to recurrent laryngeal N – horsness of sound -temporary aphonia/ dysphonation - laryngeal spasm• Recurrent laryngeal N- supply all laryngeal muscles except *cricothyroid• Nonrecurrent laryngeal N*• Injury to external laryngeal N – monotonous voice(paralysis of cricothyroid)• Inadvert removal of parathyroid gland – tetany (fatal)

- relationship between RLN and ITA highly variableExamples:• Deep to ITA (40%)• superficial (20%)• b/w branches of the artery (35%)

Anomalies of development

• Anomalies of shape• Anomalies of position• Ectopic thyroid glands• Remnants of thyroglossal duct

In agenisis of ishthmus and lobes presence of ectopic thyroid tissue must be looked for

Thyroglossal fistulaThyroglossal cyst

Thyroid Neoplasm

Benign Malignant

PrimaryFollicularPapillary

Follicular Cells

Parafollicular Cells

Lymphoid Cells

LymphomaMedullary

Differentiated Undifferentiated

AnaplasticFollicularPapillary

Hurthle Cell

Secondary