Thyroid Flashcards
Embryology of thyroid
Develops from foramen cecum (base of tongue)
Descends to anterior neck forming thyroglossal duct
Position of the thyroid
Wrapped around cricoid cartilage and superior rings of trachea C5-T1
Within visceral compartment of the neck, surrounded by pre-tracheal fascia
Two lobes
Position of the thyroid
Tow lobes
Overlying cricoid cartilage and superior rings of trachea
C5-T1
Visceral compartment of neck, bound by pretracheal fascia
Blood supply to thyroid
Superior thyroid artery (1st branch of external carotid)
Inferior thyroid artery (thyrocervical trunk, branch of subclavian)
Thyroid ima artery (in 10%, from brachiocephalic trunk)
Venous drainage of the thyroid
Venous plexus Drains into Superior thyroid vein (drains into IJV) Middle thyroid vein (drains into IJV) Inferior thyroid vein (brachiocephalic)
Lymphatic drainage of thyroid
Paratracheal nodes
Deep cervical nodes
Innervation of the thyroid
Sympathetic trunk
Anatomical relations of the thyroid
Right & left recurrent laryngeal nerves run through tracheoesophageal groove passing deep to thyroid to innervate larynx
Type of thyroid cells
Follicular cells: thyroid epithelium: simple cuboidal. Form follicles filled with colloid.
Parafollicular cells.
Function of the thyroid
Endocrine gland
Produces: T3/T4 and calcitonin
Hypothalamic-pituitary-thyroid axis
Hypothalamus detects low T3/T4 conc and releases thyrotrophin-releasing hormone (TRH) into hypophyseal portal system
TRH binds to receptors on thyrotrophic cells of ant pituitary causing them to release TSH
TSH binds to receptors on basolateral membrane of thyroid and induces synthesis and release of thyroid hormone from follicular cells
Functions of follicular cells
Effects of T3/T4
increases BMR Gluconeogenesis Glycogenolysis Lipogenesis Thermogenesis Increases expression of B receptors of cardiac tissue
Functions of parafollicular cells
Produce calcitonin
Effects of calcitonin
Inhibits Osteoclasts, reducing bone resorption
Inhibits renal tubular reabsorption of calcium and phosphate
Opposes effect of PTH
Net effect: reducing Ca2+
Thyroid cancer subtypes
Papillary carcinoma (75%) Follicular carcinoma (15%) Medullary carcinoma (3%) Anaplastic thyroid cancer (5%) Lymphoma (1-2%)
Papillary Carcinoma
Most common (75%). Most commonly 40-50 women. RFs: ionising radiation in childhood
Multiple lesions within gland. Rarely encapsulated.
Histology: papillary and colloid-filled follicles. “Orphan Annie Eye nucleus” (little protein/DNA).
Commonly spread to lymphatics
Good prognosis: 90% 10 year survival
Follicular carcinoma
2nd most common (15%). Usually seen 40-60 women. RF: low dietary iodine.
Focal encapsulated lesions.
Invades blood vessels.
Mets: maematogenous spread: Lungs, liver, bone, brain. Unlikely to invade lymph node.
Subtype: Hurthle cell tumours.
Medullary carcinoma
Rare (3%). RF: MEN2a/2b
From parafollicular cells (more commonly found in superior 1/3).
Histology: spindle shaped cells with amyloids
Raised calcitonin levels. Can secrete VIP/serotonin.
Lymphatic and medullary spread.
Poor prognosis.
Anaplastic thyroid cancer
5%. RFs: elderly
Aggressive: Rapidly growing with early local invasion.
Poor prognosis.
Signs of thyroid cancer
Single hard thyroid nodule.
Local invasion: hoarseness (larynx/RLN), dysphagia (oesophagus).
Usually non-functional (no Sx of hyper/hypothyroid).
Investigation of thyroid cancer
Initial TFTs. Calcitonin
US neck (micro calcifications, hypoechongenicity, irregular margin)
Fine needle aspiration cytology.
Management of thyroid cancer
Surg: Hemi-thyroidectomy, total thyroidectomy +/- neck dissection
Non-surg: radioiodine, external beam radiotherapy, chemo.
Parathyroid glands location
Posterior aspect of the thyroid gland
External to thyroid
Contained within pretracheal fascia
Embryology of the parathyroids
Superior parathyroids: 4th pharyngeal pouch
Inferior parathyroids: 3rd pharyngeal pouch
Blood supply to the parathyroids
Inferior thyroid arteries
collaterals from superior thyroid arteries
Venous drainage of the parathyroids
Superior, middle and inferior thyroid veins
Nerve supply to the parathyroids
Sympathetic nerves from thyroid branches
Lymphatic drainage from parathyroid
Paratracheal and deep cervical nodes
Parathyroid gland cell types
Chief cells: secrete parathyroid hormone
Oxyphil cells - unknown purpose
Actions of parathyroid hormone
Net effect: increase Ca2+
Bone: Increase resorption (indirectly acts on osteoclasts to increase breakdown of bone)
Kidney: increase Ca reabsorption, increase phosphate excretion (acts in loop of Henle and distal tubules)
Vitamin D synthesis
Activation and actions of Vitamin D
Skin: Cholecalciferol (D3)
Liver: Converted to calcifediol (25-hydroxycholecalciferol
Kidney: Converted to active form calcitriol (1,25-dihydroxycholecalciferol)
Increases absorption of Ca in GI
Increase renal tubular reabsorption of Ca
Indirectly activates osteoclasts
Types of hyperparathyroidism
Primary
Secondary
Tertiary
Malignant
Primary hyperparathyroidism
Direct alterations to parathyroid
Excess secretion of PTH
Elevated calcium
Causes: solitary adenoma, hyperplasia of glands, parathyroid cancer
Secondary hyperparathyroidism
Physiologically elevated PTH to reduced calcium levels
Causes: chronic renal failure, vitamin D def
Tertiary hyperparathyroidism
Prolonged secondary hyperparathyroidism
Glands undergo hyperplasia or adenomatous change
PTH remains high inappropriately when Ca normalised
Causes: post-renal transplant for chronic renal failure
Malignant hyperparathyroidism
Parathyroid related hormone (PTHrP) released by cancers. Mimics PTH
Raised Ca, reduced PTH (intact feedback loop)
Causes: bronchial SCC, breast ca, renal cell ca
