1. Functional Anatomy of the Thyroid & Parathyroid Glands Innervation of the Pharynx & Larynx
Dr James Peerless
June 2011

2. Objectives The Thyroid Gland The Parathyroid Gland Pharynx & Larynx
Anatomy
Endocrine function
The Parathyroid Gland
Ca2+ metabolism
Pharynx & Larynx
Innervation

3. The Thyroid Gland Largest endocrine gland in the body Origin
Originates from the floor of pharynx
Thyroglossal duct sometimes persists in adults
Largest endocrine gland in the body
Affects basal metabolic rate via the thyroid hormones: control CHO, protein metabolism, and influence growth, maturation and body temperature
Originates from floor of the pharynx
Thyroglossal duct marks path of gland from the base of the tongue to its final site

4. The Thyroid Gland Relations Deep to sternothyroid and
sternohyoid muscles
Level of C5 –T1
Bilobular; joined by the isthmus
Anterolateral to the larynx and trachea (2nd & 3rd rings)
Bound by a loose connective tissue capsule which is anchored to the cricoid cartilage and tracheal rings
Upper and lower pole

5. Supplied by superior and inferior thyroid arteries
Highly vascular
Supplied by superior and inferior thyroid arteries
Drainage via superior, middle and inferior thyroid veins
2+3
Superior thyroid artery is commonly the first branch of the external carotid
Inferior thyroid artery originates from the thyrocervical trunk (from the subclavian artery).
10% of people have the unpaired thyroid ima artery – supplied directly from the aorta or the common carotid a. – important when planning tracheostomies as it runs arteriorly along the trachea.
Sup. and middle drain into IJV, inf. into the brachiocephalic veins

6. The Thyroid Gland Nerve Supply
Derived from superior, middle and inferior cervical sympathetic ganglia (from the sympathetic trunk)
Fibres are vasomotor
No secretomotor parasympathetic innervation: the gland is hormonally regulated

7. Ectopic glands
Accessory tissue
Pyramidal lobe
Perseveration of the thyroglossal duct

8. Functional Anatomy of Thyroid
Follicular structure lined by epithelial cells
Alter shape depending on amount of activity
Contain colloid
Parafollicular ‘C’ cells
Secrete calcitonin

9. Parafollicular Cells
Thyroglobulin Colloid
Follicular Cells

10. Thyroid Hormone Synthesis
Function of the follicular cells:
Thyroglobulin production (protein containing 123 tyrosine residues)  forms the colloid
thyroid peroxidase synthesis
storage of T3 and T4
1 iodide ions actively transported into the follicular cells from the plasma – 20-40x concentration
2 iodide ions reduced to iodine in the colloid by TP and combined with tyrosine to form MIT or DIT.
3 DIT/MIT combine to form T3/T4/thyroglobulin complex.
4 passes back into the follicular cell until released into the blood.

11. The Parathyroid Glands
Four disc-like glands, embedded in the thyroid gland (dorsal surface)
Contain two cell types:
Chief cells producing PTH
Oxyphil cells: function unknown
Essential for maintenance of normocalcaemia – important as Ca2+ act as secondary messengers, blood coag., nerve impulse transmission, muscle contraction
Four disc-like glands, embedded in the thyroid gland – two in each of the upper and lower poles
Most people have 4 glands – 5% have more.
Blood supply is most commonly provided by the inferior thyroid arteries (but can be any).

12. Sites of aberrant parathyroid glandular tissue

13. Parathyroid Hormone Polypeptide produced in the chief cells
Preprohormone  Prehormone  PTH
Increases Ca2+ via three main actions:
Resorption
Reabsorption
Absorption
Resorption: mobilization of Ca2+ from bone (clasts and blasts)
Increased reabsorption from the distal tubule (and reduced reabsorption of PO42- in the proximal tubules)
Increased production of 1,25 dihydroxycholecalciferol  increased absorption of Ca2+ from the small intestine
Controlled by negative feedback loop.

14. Responds to low serum Ca2+ levels

15. The Pharynx
External circular – constrictors (superior, middle and inferior constrictors)
Internal longitudinal – elevate the larynx and shorten the pharynx during speaking and swallowing – stylopharyngeus, salpingopharyngeus, palatopharyngeus
Anterior connections such as the mandible, hyoid and larynx – and join at the posterior midline raphe.

16. The Pharynx Nerve Supply
Pharyngeal plexus provides motor and most of sensory
Motor:
All muscles – fibres from nucleus ambiguus of the accessory nerve carried via the vagus nerve
Stylopharyngeus – CN IX
Tensor veli palatini - CN V3
Lower pharynx also receives some motor fibres from recurrent laryngeal nerve
Sensory:
Glossopharyngeal (CN IX) – most of mucosa
Maxillary (CN V2) – anterior nasopharynx
Vagus (CN X)
Branches from the cervical sympathetic chain

17. The Larynx Muscles (6+3) Intrinsic Extrinsic Cricothyroid
Posterior cricoarytenoid
Lateral cricoarytenoid
Transverse arytenoid
Aryepiglottic
Thyroarytenoid
All paired except the tranverse arytenoid
Extrinsic
Sternothyroid
Thyrohyoid
Inferior Constrictor

18. The Larynx Innervation Mucous membranes Muscles of the larynx
Above cords: internal laryngeal nerve
Below cords: recurrent laryngeal nerve
Muscles of the larynx
Cricothyroid: superior/external laryngeal nerve
All others: recurrent laryngeal nerve

19. Objectives The Thyroid Gland The Parathyroid Gland Pharynx & Larynx
Anatomy
Endocrine function
The Parathyroid Gland
Ca2+ metabolism
Pharynx & Larynx
Innervation

20. Questions

21. Reference
Smith T, Pinnock C, Lin T (2009). Fundamentals of Anaesthesia, 3rd Edition. Cambridge: Cambridge University Press
Moore K, Dalley A (1999). Clinically Oriented Anatomy, 4th Edition. Philadelphia: Lippincott Williams & Wilkins