Thyroid Gland anatomy & investigations Flashcards
What does the thyroid gland develop from?
the endodermal diverticulum in the middle of the primitive pharynx (FORAMEN CAECUM of tongue)
Describe the development of the thyroid gland?
the diverticulum elongates forming the thyroglossal duct —> thyroglossal duct descends caudally in front of the primitive pharynx and in front of hyoid bone —> duct bifurcates at the level of the thyroid cartilage to form lobes and isthmus and pyramidal lobe
What is responsible for the development of the most lateral parts of the lateral lobes of thyroid gland?
from the 4th pharyngeal arch ULTIMOBRANCHIAL BODY
What thyroid cells are derived from the neural crest? and what is their function?
parafollicular C cells
secretion of calcitonin
Where is the thyroid located?
lower part of the neck in the muscular triangle infront of sternomastoid muscle
where are the right and left lateral lobes located?
extend from the oblique line of the thyroid cartilage to the 6th tracheal ring
where is the isthmus located?
connects the 2 lateral lobes in midline extending from 2nd to 4th tracheal rings
What are the medial relations of the thyroid glands?
- upper part: larynx & pharynx with superior laryngeal nerve in between
- lower part: trachea & esophagus with recurrent laryngeal nerve
What are the posterolateral relations of the thyroid?
Carotid sheath & its contents
- artery CAROTID ARTERY
- vein INTERNAL JUGULAR VEIN
- 2 nerves CERVICAL 1 SYMPATHETIC CHAIN & VAGUS NERVE
Superior & inferior parathyroid glands & muscles
What is the superficial relation of the thyroid gland?
skin platysma sternomastoid STRAP MUSCLES - sternohyoid - sternothyroid - omohyoid
What is the recurrent laryngeal related to?
Berry’s ligament: condensation of pretracheal fascia forming the ligament
during surgery be careful not to injure the recurrent laryngeal nerve next to it
What muscles does the superior laryngeal nerve supply?
it gives the external laryngeal branch which supplies the cricothyroid muscle responsible for HIGH PITCH
accompanies superior thyroid artery
the superior laryngeal nerve originates from which nerve?
Vagus nerve
The internal laryngeal nerve supplies?
mucous membranes above vocal cords
What does the recurrent laryngeal nerve supply?
motor: all internal laryngeal muscles except cricothyroid
sensory: mucous membranes below vocal cords
What are the coverings of the thyroid gland?
- true capsule surrounding the thyroid gland itself
- false capsule surrounding the whole thyroid formed from PRETRACHEAL FASCIA is attached to the hyoid bone superiorly
What is the blood supply of the thyroid artery?
- SUPERIOR THYROID ARTERY
(accompanying the external laryngeal nerve)
FROM external carotid artery - INFERIOR THYROID ARTERY
(accompanying recurrent laryngeal nerve)
FROM thyrocervical trunk of the 1st part of subclavian - THYROID IMA branch of aorta or brachiocephalic
- TRACHEAL AND OESOPHAGEAL BRANCHES
to retain thyroid gland after thyroidectomy
What is the venous drainage of the thyroid gland?
SUPERIOR THYROID VEIN -> drains into the internal jugular
MIDDLE THYROID VEIN -> short & drain into internal jugular
INFERIOR THYROID VEIN -> tracheal & esophageal branches
What are the 2 secretory cells of the thyroid gland?
- follicular cells: T3 & T4
- Parafollicular cells (C cells): calcitonin
What are the lymph nodes that drain the thyroid?
- prelaryngeal
- pretracheal
- paratracheal
they all drain into superior & inferior deep cervical nodes
What are the mechanisms of thyroid hormone synthesis??
1- IODINE TRAPPING: inorganic iodide is picked up by thyroid under the influence of TSH
2- OXIDATION: perioxidaze turns it to organic iodine
3- BINDING: organic iodine with tyrosine forming iodothyronins (MIT, DIT)
4- COUPLING: of MIT & DIT forming T3 & T4
5- RELEASE: by splitting thyroglobulin by protease
T3 & T4 bind to albumin & globulin but a small portion is free and physiologically active
What are the functions of thyroxin?
- stimulate O2 consumption (energy production)
- stimulate glucose utilization by tissue
- enhance protein catabolism
- decrease serum cholesterol level
- sensitize heart & CNS to catecholamines
- stimulate physical mental and sexual growth in children
Whats the function of calcitonin?
lowering the serum calcium level (antagonist for parathormone)
What changes will occur in the hormone levels incase of hyperthyroidism & hypothyroidism?
Hyperthyroidism Hypothyroidism
T3 increase decrease
T4 increase decrease
TSH decrease increase
What is the most important investigation for follow up?
THYROID FUNCTION TEST (TSH)
- free is more sensitive than total
- can differentiate between high normal and abnormally high thyroid hormones
from 4000 - 11000 NORMAL
What condition is caused due to high T3 & normal T4?
T3 thyrotoxicosis
What investigation is used to diagnose Grave’s disease or autoimmune thyroiditis?
AUTOANTIBODY TITERS
What are the types of autoantibodies?
- LATS (trab)
TSH receptor antibody: bind to TSH receptors and increase stimulation causing GRAVES - Anti PO, antimitochondrial, antithyroglobulin
inhibitors of thyroid hormone formation cause HASHIMOTO
What is the most important method of investigation to differentiate between a toxic nodule and a solitary nodule of toxic nodular goiter?
isotope scan
- hot nodule takes up isotope but surroundings don’t
- warm nodules takes up isotope and surroundings do
- cold nodules do not take up isotope
What is the first line method of radiological investigation in case of suspected thyroid disease?
ultrasonography
- detect enlargement of thyroid gland
- comment on any nodules or cysts
- guide FNAC
- asses malignant lymphadenopathy
How to differentiate between a benign or malignant cyst in the thyroid?
BENIGN MALIGNANT
- smooth - rough
- regular - irregular
- clear content - turbid content
- serous fluid - bloody fluid
- no residual mass - residual mass after aspiration
after aspiration
- no or slow recollection - rapid recollection
- no malignant cells - malignant cells
What are the important TIRADS scores?
TR 1 = no FNA
TR 2 = no FNA
TR 3 = monitor if > or = to 1.5cm
= FNA if > or = to 2.5cm
TR 4 = follow if > or = to 1cm
= FNA if > or = to 1.5cm
TR 5 = follow if > or = 0.5cm
= FNA if > or = to 1cm
What is the investigation of choice in discrete thyroid swellings?
FNAC
- can diagnose colloid nodules, thyroiditis, papillary carcinoma. medullary carcinoma, anaplastic carcinoma & lymphoma
- CAN’T distinguish between follicular adenoma or carcinoma
When is indirect laryngoscopy used?
to assess vocal cords before surgery
What method of investigation is best for assessing retrosternal & recurrent swellings?
CT & MRI
What is the importance of neck X-ray?
demonstrates
- tracheal deviation
- compressions
- retrosternal extension
- ring or rim calcification
Which method of investigation is malignancy specific?
PET scan
localizes disease that doesn’t take up radioiodine
What is the most confirmatory line of investigation in a solitary thyroid nodule?
FNABC
during thyroidectomy, surgeon should consider special care during ligation of superior thyroid artery as he may affect?
cricothyroid muscle supplied by the external laryngeal nerve
