Surgical anatomy and physiology of thyroid and parathyroid

1. BY DR. AJAY KUMAR
1ST YEAR SURGERY PG
MKCG MCH

2. AIM OF THE TOPIC
 To know the development and associated anomalies of
thyroid & parathyroid gland.
 To know the basic anatomy, relations, blood supply,
lymphatic drainage, nerve supply of thyroid &
parathyroid gland & its surgical importance.
 To know the physiology of thyroid & parathyroid
gland.

3. Development of Thyroid gland
 The thyroid gland appears as epithelial proliferation in the foramen
caecum between tuberculum impar & the copula. The vestigial
marking of thyroglossal duct is the foramen caecum of tongue.
 Subsequently it descends downward in front of pharyngeal gut as a
bilobed diverticulum.
 During this migration, the thyroid remains connected to the tongue by
a narrow canal called thyroglossal duct which later disappear.
 It reaches its final position in front of trachea in the 7th week of IUL.
By then it has acquired a small median isthmus & two lateral lobes.
 Thyroid begins to function at approx. the end of 3rd month of IUL.
 Follicular cells produces the colloid that serves as a source of thyroxin &
T3. Para follicular or “ C” cells derived from ultimobranchial body
serves as a source of calcitonin.

5. Developmental Anomalies
 Thyroglossal cyst : It is the cystic remnant of Thyroglossal
duct. It may lie at any point along the migratory pathway of thyroid
gland but is always near or in the midline of the neck.
 Approximately 50% of these cysts are close to or just inferior to body of
hyoid bone, they may also be found at the base of tongue or close to
thyroid cartilage.
 Thyroglossal Fistula : Usually arises secondarily after
rupture of thyroglossal cyst, but may be present at birth..
 Aberrant thyroid tissue : May be found anywhere along
the path of descend of the thyroid gland. It commonly found in the
base of tongue called lingual thyroid just behind foramen caecum.

6. Throglossal cyst & fistula

8. Basic anatomy of thyroid

9.  Thyroid means “ shield like “ . It is an endocrine gland situated in the
lower part of front and side of neck.
 It consists of right & left lobes which is joined together by isthmus,
sometimes pyramidal lobe may be present.
 Thyroid gland lies against C6 to T1 vertebrae and embracing the upper
part of trachea.
 Each lobes extends from middle of thyroid cartilage to 4th tracheal ring,
isthmus extends from 2nd to 4th tracheal ring.
 Dimension : Each lobe – 5x2.5x2.5 cm, isthmus – 1.2x1.2cm.
 Weight : 25gm, larger in female & enlarges during menstruation &
pregnancy.
 Capsule of thyroid gland : True & False capsule.
True capsule is the peripheral condensation of connective tissue of
gland. A dense capillary plexus is present deep to true capsule, so to
avoid hemorrhage during surgery thyroid is removed along with true
capsule.
False capsule is derived from pretracheal layer of deep cervical fascia.

10. Parts & Relations
LOBES : Lobes are conical in shape and having –
- Apex
- Base
- Three surfaces – lateral, medial, posterior
- Two borders – anterior & posterior.
Apex : Directed upward & slightly laterally. It is limited superiorly by
attachment of sternothyroid muscle. It is related to superior thyroid artery
and external laryngeal nerve.
Base : At the level of 4th or 5th tracheal ring. It is related to inferior thyroid
artery & recurrent laryngeal nerve.
Lateral surface : It is convex & covered by :-
- Sternohyoid
- Superior belly of omohyoid
- Strernothyroid
- Ant. Border of SCM muscle

11. Medial surface : It is related with :
Two tubes – Trachea & Esophagus.
Two muscle – Inferior constrictor & Cricothyroid.
Two nerves – Ext. laryngeal nerve & Recurrent laryngeal nerve.
Posterior surface : Related to carotid sheath with vessels & nerve.
Anterior border : Related to ant. branches of superior thyroid artery.
Posterior border : It is related with –
- inferior thyroid artery
- anastomosis between post. branch of sup. thyroid artery &
ascending branch of inferior thyroid artery.
- parathyroid gland
- thoracic duct on left side.
ISTHMUS : Connects lower parts of two lobes.
It has : Two surfaces – anterior & posterior
Two borders - upper & lower.

12. Anterior surface : It is covered by :
- Rt. & Lt. sternothyroid & sternohyoid muscle.
- Ant. Jugular vein.
- Fascia & skin.
Posterior surface : It is related with 2nd to 4th tracheal ring.
Upper border : It is related with anterior branch of right & left superior
thyroid artery & anastomosis.
Lower border : It is related to inferior thyroid vein.

14. Arterial Supply of Thyroid
 Superior thyroid artery : -
- It is the 1st anterior branch of external carotid artery.
- It is related to external laryngeal nerve.
- At the upper pole give 2 branches : anterior & posterior branch.
Anterior branch descend on anterior border of lobe and continues along
upper border of isthmus & anastomoses with its fellow of opposite side.
Posterior branch descends on posterior border of the lobe & anastomoses
with ascending branch oh inferior thyroid artery.
 Inferior thyroid artery :-
- It is a branch of thyrocervical trunk.
- It runs first upward, then medially and finally downward to reach the
base of the gland.
- It divides into 4 to 5 glandular branches which pierces the fascia to reach
the lower part of the gland.

15. - One ascending branch anastomoses with posterior branch of
superior thyroid artery & supplies the parathyroid gland.
 Thyroidea Ima artery :-
- It is present in only 3% individual.
- It arises from Bracheocephalic trunk or directly from Arch of
Aorta.
- It enters the lower part of isthmus.
 Accessory thyroid artery :-
- It arises from tracheal & esophageal artery and also supplies the
thyroid gland.

19. Venous Drainage of Thyroid
 Superior Thyroid Vein :-
- It emerges at upper pole of thyroid gland.
- It accompanies superior thyroid artery.
- It ends by draining into Internal Jugular vein.
 Middle Thyroid Vein :-
- It emerges at middle of the lobe of thyroid gland.
- It also ends by draining into IJV.
- It is first vessel to be ligated in thyroid surgery.
 Inferior Thyroid Vein :-
- It emerges at lower border of isthmus.
- It forms a plexus in front of trachea.
- It drains into Bracheocephalic vein.
 Vein of Kocher :- Sometimes present & drains into IJV.

22. LYMPHATIC DRAINAGE OF THYROID
Upper part :- Drains into upper deep cervical lymph node either
directly or through prelaryngeal nodes ( Delphian nodes ).
Lower part :- Drains into lower deep cervical lymph node directly and
also through pretracheal ( Sub- Delphian nodes ) & para tracheal
lymph nodes.
NERVE SUPPLY OF THYROID GLAND
-Nerves are mainly derived from middle cervical ganglion
and partly from superior & inferior cervical ganglion.
- These are vasoconstrictor.

23. Surgical importance of
nerves & arteries
 Recurrent Laryngeal nerve :-
- The recurrent laryngeal nerve ascends on either sides of
trachea & lies just lateral to ligament of Berry as it enters the
larynx.
- In 25% of patients, the Recurrent laryngeal nerve is contained
within the ligament as it enters the larynx.
- Rt. Recurrent Laryngeal nerve originates from Vagus nerve
as it crosses the Subclavian artery, then it passes posterior to the
Subclavian artery & ascends lateral to trachea along the
tracheoesophageal groove.
- At midportion of thyroid, the nerve may divide into one, two,
or more branches. The nerve can usually be found immediately
anterior or posterior to a main arterial trunk of inferior thyroid
artery at this level.

24. - A nonrecurrent right laryngeal nerve can arise directly
from the vagus & course medially into the larynx. This
occurs in 0.5 to 1.5% of the patients & occurs in the setting
of arterial anomalies ( arteria lusoria ).
- Left side, the Recurrent Laryngeal nerve separates from
vagus as that nerve traverses the Arch of Aorta.
- It passes inferiorly & medially to the Aorta at the
ligamentum arteriosum & begins to ascends towards the
larynx, where it enters the tracheoesophageal groove as it
ascends to the level of lower lobe of thyroid.
- A nonrecurrent left laryngeal nerve is associated with
more extensive anomalies and it is very rare.

26.  Superior Laryngeal Nerve :-
- It separates from the vagus nerve at the base of the skull
and descends toward the superior pole of the thyroid along
the internal carotid artery
- At the level of the hyoid cornu, it divides into external
and internal branches
- External branch continues to travel along the lateral
surface of the inferior pharyngeal constrictor muscle and
usually descends anteriorly and medially, along with the
superior thyroid artery
- Within1 cm of the entrance of the superior thyroid
artery into the thyroid capsule, the nerve generally takes a
medial course and enters the cricothyroid muscle
- Damage to the external branch can result in severe loss
of voice quality or strength.

27.  Cernea’s classification :-
According to this external branch of superior laryngeal nerve is
divided into type 1, type 2A and 2B.
 Space of Reeves :-
- It is the avascular space between the upper pole of thyroid and
cricothyroid muscle.
- It is crucial for safe dissection.
- This is critical step in order to avoid damage to external
laryngeal nerve.
 Tubercle of Zuckercandl :-
- It is the posterior extension of lateral lobes.
- It is found in 60 - 70% of people and whenever found ,
recurrent laryngeal nerve is usually deep to it.
- It is more common on right side.

28. Space of Reeves

29. Tubercle of Zuckercandl

30. Physiology of thyroid
hormone
The thyroid gland synthesizes and secretes three
hormones:
• Thyroxine(T4).
• Tri- iodothyronine(T3).
• Calcitonin

31. Regulation
Hypothalamic thyrotropin-releasing hormone (TRH)
stimulates the release of thyroid-stimulating hormone
(TSH) from thyrotrophs in the anterior pituitary gland
and also causes upregulation of TSH gene
transcription.
This causes increase release of thyroid hormones
from thyroid gland

32. Thyroxine (T4) and Tri-iodothyronine(T3).
 Both of these hormones profoundly increase the metabolic
rate of the body.
 Complete lack of thyroid secretion usually causes the basal
metabolic rate to fall 40 to 50 per cent below normal, and
extreme excesses of thyroid secretion can increase the
basal metabolic rate to 60 to 100 per cent above normal.
 Thyroid secretion is controlled primarily by thyroid-
stimulating hormone (TSH) secreted by the anterior
pituitary gland.

33.  About 93 per cent of the metabolically active hormones
secreted by the thyroid gland is thyroxine, and 7 per cent
tri-iodothyronine.
 However, almost all the thyroxineis eventually converted to
tri-iodothyronine in the tissues, so that both are
functionally important.
 The functions of these two hormones are qualitatively the
same, but they differ in rapidity and intensity of action.
 Tri-iodothyronine is about four times as potent as
thyroxine, but it is present in the blood in much smaller
quantities and persists for a much shorter time than does
thyroxine.

34.  The thyroid gland is composed of large numbers of closed
follicles (100 to 300 micrometers in diameter) filled with a
secretory substance called colloid and lined with cuboidal
epithelial cells that secrete into the interior of the follicles.
 The major constituent of colloid is the large glycoprotein
thyroglobulin, which contains the thyroid hormones
within its molecule.
 The thyroid gland has a blood flow about five times the
weight of the gland each minute, which is a blood supply as
great as that of any other area of the body
 To form normal quantities of thyroxine, about 50
milligrams of ingested iodine in the form of iodides are
required each year. The minimum daily iodine intake that
will maintain normal thyroid function is 150 micrograms in
adults.

35. Thyroid Hormones
synthesis & secretion
 Iodine is an essential raw material for thyroid
hormone synthesis. Dietary iodide is absorbed by the
intestine and enters the circulation.
 The basolateral membranes of thyrocytes facing the
capillaries contain a symporter that transports two
Na+ ions and one I– ion into the cell with each
cycle, against the electrochemical gradient for I–.
 This Na+/I– symporter(NIS)is capable of producing
intracellular I– concentrations that are 20 to 40
times as great as the concentration in plasma.

36.  The interface between the thyrocyte and the colloid, iodide
undergoes a process referred to as organification.
 The oxidation and reaction of iodide with the secreted
thyroglobulin is mediated by thyroid peroxidase, a
membrane-bound enzyme found in the thyrocyte
apical membrane.
 When there is a need for thyroid hormone secretion,
colloid is internalized by the thyrocytes by endocytosis,
and directed toward lysosomal degradation.
 Thyrocytes function can be summed-up in this way: they
collect and transport iodine, they synthesize thyroglobulin
and secrete it into the colloid, they fix iodine to the
thyroglobulin to generate thyroid hormones, and they
remove the thyroid hormones from thyroglobulin and
secrete them into the circulation

37.  Thyroid hormone synthesis is a multistep process.
 There are two theories of how this coupling reaction
occurs. One holds that the coupling occurs with both
DIT molecules attached to thyroglobulin
(intramolecular coupling)
 The other holds that the DIT that forms the outer ring is
first detached from thyroglobulin (intermolecular
coupling)
 T3 is formed by condensation of MIT with DIT. A small
amount of RT3 is also formed, probably by condensation of
DIT with MIT.
 In the normal human thyroid, the average distribution of
iodinated compounds is 23% MIT, 33% DIT, 35% T4, and
7% T3. Only traces of RT3 and other components are
present.

39. Mechanism of thyroid hormones
 Thyroid hormones enter cells and T3 binds to thyroid
receptors (TR) in the nuclei. T4 can also bind, but not
as avidly.
 The hormone-receptor complex then binds to DNA via
zinc fingers and increases (or in some cases, decreases)
the expression of a variety of different genes that code
for proteins that regulate cell function

40. Physiological effect of Thyroid
hormone in brief

41. Calcitonin
 Calcitonin (also known as thyrocalcitonin) is a 32-
amino acid linear polypeptide hormone
 Calcitonin was purified in 1962 by Copp and Cheney.
 While it was initially considered a secretion of the
parathyroid glands it was later identified as the secretion of
the C-cells of the thyroid gland
 It acts to reduce blood calcium, opposing the effects of
PTH
-Inhibits Ca2+absorption by the intestines
-Inhibits osteoclast activity in bones
-Inhibits renal tubular cell reabsorption of Ca2+ allowing
it to be excreted in the urine.

42. PARATHYROID GLANDS

43. Development of Parathyroid
 Inferior Parathyroid develops from 3rd & superior
Parathyroid develops from 4th pharyngeal pouch
respectively.
 Primordia of inferior parathyroid along with
primordia of thymus migrate downward & finally
comes to rest on inferior part of dorsal surface of
thyroid gland.
 Primordia of superior parathyroid rests on
superior part of dorsal surface of thyroid gland.

45. Basic Anatomy
 Parathyroid glands are 2 pairs in number.
 The parathyroid glands are small, yellowish-brown,
ovoid or lentiform structures, usually lying between
the posterior lobar borders of the thyroid gland and its
capsule.
 Size – 6x4x2 mm.
 Weight – Each weighs 50 micrograms.
 The superior parathyroid glands are more constant in
location than the inferior and are usually to be found
midway along the posterior borders of the thyroid
gland, although they may be higher.

46.  The inferior pair are more variably situated and may be
within the fascial thyroid sheath, below the inferior
thyroid arteries and near the inferior lobar poles; or
outside the sheath, immediately above an inferior
thyroid artery; or in the thyroid gland near its inferior
pole.
 The superior parathyroids are usually dorsal, the
inferior parathyroids are ventral, to the recurrent
laryngeal nerves.

48. VASCULAR SUPPLY AND LYMPHATIC
DRAINAGE OF PARATHYROID
 The parathyroid glands have a rich blood supply from
the inferior thyroid arteries or from anastomosis
between the superior and inferior thyroid vessels.
 Approximately one-third of human parathyroid glands
have two or more parathyroid arteries.
 Lymph vessels are numerous and associated with those
of the thyroid and thymus glands.

49. NERVE SUPPLY OF PARATHYROID GLAND
 The nerve supply is sympathetic, either direct from the
superior or middle cervical ganglia or via a plexus in
the fascia on the posterior lobar aspects.
 Parathyroid activity is controlled by variations in
blood calcium level: it is inhibited by a rise and
stimulated by a fall in serum Calcium level.
 The nerves are believed to be vasomotor but not
secretomotor

50. PARATHYROID HORMONE ( PTH )
 Parathyroid hormone (PTH) or parathormone or
parathyrin, is secreted by the chief cells of the
parathyroid glands
 It acts to increase the concentration of calcium(Ca2+)
in the blood,
 PTH half-life is approximately 4 minutes
 The average PTH level is 10-60 pg/ml.

51. SYNTHESIS OF PTH
 PTH is a linear polypeptide with a molecular weight of 9500
that contains 84 amino acid residues .
 It is synthesized as part of a larger molecule containing 115
amino acid residues called preproPTH.
 On entry of preproPTH into the endoplasmic reticulum, a
leader sequence is removed from the amino terminal to form
the 90-amino-acid polypeptide proPTH.
 Six additional amino acid residues are removed from the
amino terminal of proPTH in the Golgi apparatus, and the
84-amino-acid polypeptide PTH is packaged in secretory
granules and released as the main secretory product of the
chief cell.

53. MECHANISM OF ACTION OF PTH
 There are at least three different PTH receptors. One
binds parathyroid hormone-related protein (PTHrP)
and is known as the PTHrP receptor.
 A second receptor, PTH2 (hPTH2-R), does not bind
PTHrP and is found in the brain, placenta, and
pancreas.
 The third receptor, CPTH, which reacts with the
carboxyl terminal rather than the amino terminal of
PTH.

55. REGULATION OF SECRETION OF PTH
 Circulating ionized calcium acts directly on the
parathyroid glands in a negative feedback fashion to
regulate the secretion of PTH.
 The key to this regulation is a cell membrane Ca2+
receptor, CaR.
 Activation of this G-protein coupled receptor leads to
inhibition of PTH secretion in parathyroid gland.
 In this way, when the plasma Ca2+ level is high, PTH
secretion is inhibited and the Ca2+ is deposited in the
bones. When it is low, secretion is increased and Ca2+
is mobilized from the bones.

56. FUNCTION OF PTH
 Parathyroid hormone raises blood Ca by acting on 3
organs:
-Bone: main effect-stimulates osteoclasts -> bone breaks
down -> Ca released.
-Intestines: increases uptake of Ca from intestine.
-Kidney: stimulates reabsorption of Ca from the kidney
tubules
 PTH reduces the reabsorption of phosphate from the
proximal tubule of the kidney which means more
phosphate is excreted through the urine
 PTH increases the activity of 1-α-hydroxylase enzyme,
which converts 25-hydroxycholecalciferol to 1,25-
dihydroxycholecalciferol, the active form of vitamin D.

57.  Stimulator of PTH Secretion : -
- Decreased serum Ca level.
- Mild decrease in serum Mg level.
- Increase in serum phosphate level.
 Inhibitors of PTH Secretion :-
- Increase serum Ca level.
- Severe decrease in serum Mg level which also
produces symptoms of hypoparathyroidism.

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