Week 3 Flashcards
What is the functional unit of thyroid gland
Follicle
Describe the structure of a thyroid follicle
Follicular cells arranged into a sphere shape.
Colloid in the centre lumen
Parafollicular cells (C cell) between follicles
Highly vascularised
What is colloid rich of
Thyroglobulin
What type of epithelial cells are follicular cells
Cuboidal cells
Describe the Hypothalamus-pituitary-thyroid axis
- Paraventricular nucleus of hypothalamus stimulated, releases TRH into hypophyseal portal system
- TRH travels to the anterior pituitary gland which stimulates the release of TSH
- TSH travels to thyroid gland through bloodstream and stimulates the synthesis and release of T3 and T4
- T3 and T4 hormones exhibit negative feedback which inhibits the release of TRH and TSH
Which part of the thyroid gland is covering the second tracheal ring
Isthmus
What structures are visible at the posterior aspect of the thyroid gland
Parathyroid glands
What would the TSH/TRH levels be in a hyperactive thyroid gland
Low due to high levels of T3 and T4
What substance is required for the synthesis of thyroid hormones
Iodine
Describe the movement of the thyroid gland from back of the tongue to its current position in an embryo
- Begins at 4th week of embryogenesis. Midline thickening at the back of the tongue
- Moves downwards, eventually reaching in front of the tracheal rings
- becomes in contact with the parathyroid glands which had developed in the area
- The residue of the thyroid gland at the back of the tongue becomes foramen caecum
Describe the development of the thyroid gland
- Endodermal thickening
- migrates downwards to form thyroglossal duct
- the 2 lobes of thyroid gland and the isthmus develop from the thyroglossal duct
- Thyroglossal duct is degraded in most people but not all
- the remnant of thyroglossal duct = pyramidal lobe
When will primitive follicles of the thyroid gland become visible and start trapping iodine
By 12 weeks
Which condition are people at risk of due to thyroglossal duct not disintegrating
Thyroglossal cysts
Where are thyroglossal cysts mostly found
Near the hyoid bone
Where to palpate the thyroid gland
- starting from the chin and moving downwards, palpate the midline
- palpate the first hard structure: thyroid cartilage
- palpate the second hard structure: cricoid cartilage
- move downwards to palpate the first.2 tracheal rings
- the isthmus overlies the second tracheal ring
- go laterally and medially around and upwards
Nerve supply of the thyroid gland
Sympathetic nerve supply: superior, medial, inferior sympathetic trunk
Parasympathetic nerve supply: Vagus nerve
Arterial supply of the thyroid gland
Superior and inferior thyroid arteries
Venous drainage of the thyroid gland
Superior and middle thyroid veins drain into the internal jugular vein
Inferior thyroid vein drains into the brachiocephalic vein
Which ligament is the primary fixation of the thyroid gland to its surrounding structures
Posterior suspensory ligament (Berry ligament)
What nerve is behind the posterior suspensory ligament
Recurrent laryngeal nerve
What structures are innervated by the recurrent laryngeal nerve
All Intrinsic muscles of the larynx except the cricothyroid muscles
Name the intrinsic muscles of the larynx innervated by the RLN
Thyroarytenoid muscle
Posterior cricoarytenoid muscle
Lateral cricoarytenoid muscle
Transverse and oblique cricoarytenoid muscles
Which nerve innervates the cricothyroid muscle
Superior laryngeal nerve - a branch of vagus nerve
Function of the cricothyroid muscle
Stretches and tenses the vocal cords to make forceful voice
Changes tone of voice
Function of the thyroarytenoid muscle
Relax the vocal cords to make softer voice
Function of the posterior cricoarytenoid muscle
Sole muscle that abducts the vocal cords hence opening the rima glottidis
Function of the lateral cricoarytenoid muscle
Major muscle that adducts the vocal cords hence narrowing the rima glottides
Function of the transverse and oblique cricoarytenoid muscles
Adducts the arytenoid cartilage to narrow the laryngeal inlet
Consequence of damage to recurrent laryngeal nerve after thyroidectomy
Hoarseness of voice
Loss of voice
Change in pitch
Dyspnea
Dysphagia
Why can dyspnea occur due to RLN damage
Because it can cause the paralysis of posterior cricoarytenoid muscle which is the only muscle that can abduct the vocal cords to open the rima glottidis
What is thyroglobulin rich of
tyrosine amino acids
Steps of thyroid hormone synthesis
- Thyroglobulin synthesised by follicular cells
- uptake and concentration of iodide into the colloid
- Iodide moves into follicular cells
- Oxidation of 2 iodide ions into iodine in the follicular cells
- Iodine passed into the colloid
- iodination of thyroglobulin
Describe the iodination of thyroglobulin to form T3 and T4
- Peroxidase enzyme in colloid links iodine to tyrosine amino acids in thyroglobulin
- Forms 2 intermediaries: tyrosine + 1 iodine and tyrosine + 2 iodine
T3 is formed by those 2 intermediaries joining together
T4 is formed by linkage of 2 tyrosine + 2 iodine
Where is thyroglobulin stored in till it is needed
Colloid
Which thyroid hormone is more potent
T3
Which drugs are used to treat hyperthyroidism
Carbimazole
Propylthiouracil
Mechanism of action of carbimazole
Inhibits thyroid peroxidase enzymes which are required for iodination of thyroglobulin
Which thyroid hormone makes up 90% of the thyroid hormones secreted
T4
Which thyroid hormone is biologically active
T3
Why does T4 need to be converted to T3
To become metabolically active
Which organs are responsible for the conversion of T4 into T3
Liver and kidney
How are thyroid hormones transported to target cells
Bound to serum proteins
What are the serum proteins that thyroid hormones can bind to
Thyroxine binding globulin
TTR (transthyretine)
Thyroxine binding pre-albumin
Albumin
Are most thyroid hormones unbound or bound to serum proteins
Bound to serum proteins
Only 0.015% of total T4 are free T4
Only 0.33% of total T3 are free T3
Bound / unbound thyroid hormones can enter the cells
Unbound
Why does T3 have a more rapid onset and offset of actions
Because it is bound less strongly by TBG (thyroxine binding globulin) than T4 and it is not bound significantly by TTR (transthyretin; transports thyroxine and vitamin A aka retinol)
Which, unbound or bound thyroid hormone concentration, correlates more closely to the metabolic state
Unbound
Effects of thyroid hormones
Increase metabolic rate
Increase in gluconeogenesis and glycogenolysis
Increase in lipolysis
Increase in respiratory rate and heart rate
Growth and development
Why does thyroid hormones cause an increase in gluconeogenesis and glycogenolysis
To increase the amount of glucose available for respiration; since increase in metabolic rate = increase in respiration = increase in glucose substrate needed
Why does thyroid hormone cause an increase in respiratory and heart rate
To meet the increase in demand of O2 due to the increase in metabolic rate
Thyroid hormones metabolic effects
Increases metabolic rate and thermogenesis
Increase blood glucose
Increase lipolysis
Increase FFA oxidation
Increase in protein synthesis
Why do thyroid hormones have effects on growth and development
Because the GHRH (growth hormone releasing hormone) and GH requires presence of thyroid hormone for activity
What can occur if a child has hypothyroidism
Growth retardation
What happens to the development of CNS in the child if the mother has hypothyroidism
The baby can have slower congnitive and intellectual functions
What enzymes are responsible in degrading t3 and T4
D1, D2, D3
Where is D3 found
Fetal tissue
placenta
Brain except pituitary
Which thyroid degrading enzyme is found in the pituitary
D2
Majority of T3 and T4 are degraded by which enzyme
D3
Causes of hypothyroidism
Primary thyroid gland failure
Secondary to TRH or TSH deficiency
Lack of iodine in diet
What is goitre
Enlargement of the thyroid
Symptoms and signs of hypothyroidism
Weight gain (due to decrease in BMR)
Slow heart rate
Fatigue
Cold intolerance
Goitre
Constipation
Dry hair
Brittle nails
Vitiligo
Mneumonic for symptoms of hypothyroidism (MOM’S SO TIRED)
Memory loss
Obesity (weight gain)
Menorrhagia
Slowness mentally and physically
Skin and hair dryness
Onset is gradual
Tiredness
Intolerance to cold
Raised BP
Enlarged thyroid gland
Depression
What can occur in babies due to hypothyroidism
Dwarfism
Intellectual defects
Slow growth and development (reach milestones later or not at all)
What is Grave’s disease
Autoimmune disease that causes hyperthyroidism
Symptoms and signs of hyperthyroidism
Sweating
Bulging eyes (proptosis)
Goitre
Weight loss
Insomnia
increased nervousness and excessively emotional
Fast heart rate
Heat intolerance
What would the thyroid hormones level be in primary hypothyroidism
Low T3/T4
High TSH
What would the thyroid hormones level be in secondary hypothyroidism
Low T3/T4
Low/normal TSH
Why is TSH level called “inappropriately normal” in secondary hypothyroidism
In secondary hypothyroidism, the low T3/T4 level should cause a high TSH level but because the secondary hypothyroidism is caused by a defect in the pituitary gland, TSH level is low/normal
What will the thyroid hormone level in secondary hyperthyroidism be
High T3/T4
High or normal TSH
Hypothyroidism is more common in which type of people
Females
White populations
People in areas with high iodine intake
What is secondary hypothyroidism
Hypothyroidism not due to thyroid dysfunction
Goitrous causes of primary hypothyroidism
Hashimoto’s thyroiditis
iodine deficiency
Amiodarone induced hypothyroidism
Non-goitrous causes of primary hypothyroidism
Atrophic thyroiditis
Congenital hypothyroidism
Post partum thyroiditis
What is Hashimoto’s thyroiditis
it is an autoimmune destruction of thyroid gland causing reduction in thyroid hormone production
Which group of people are more at risk of Hashimoto’s thyroiditis
Family history of autoimmune conditions (e.g. T1 diabetics)
Family history of autoimmune thyroiditis (grave’s / Hashimoto’s)
Turner’s and Down syndrome
Males