Week 3 - B - Week 3 - physiology and anatomy of the thyroid gland Flashcards
What is the function of the pineal gland?
The pineal gland produces melatonin, which helps maintain circadian rhythm (sleep is a circadian rhythm) and regulate reproductive hormones.
What are the three parts of the thyroid gland?
Righ and left lobe
And an isthmus connecting the two
What are the glands that lie posterior to the thyroid gland? (there are 4) What does this gland secrete?
Parathyroid glands
Secrete parathyroid hormone
Where does the thyroid gland lie?(between which vertebra/tracheal rings
The thryoid gland is normally lies from the 5th cervical to 1st thoracic vertebra
The 2nd - 4th tracheal rings
What is the approximate weight of a normal thyroid gland in adults? When can the gland physiologically increase in size?
25-30g
During pregnancy / menstruation
What is the arterial supply to the thyroid gland? What are the artieries branches off? What is the third artery to supply the thyroid that is not always present?
Superior thyroid artery - branch of external carotid artery
Inferior thyroid artery - branch of thyrocervical trunk from the subclavian artery
The third artery is the thyroid ima artery
What is the venous drainage of the thyroid gland? Where do the veins drain?
Superior thyroid vein and middle thyroid vein drain to the internal jugular veins
Inferior thyroid vein - drains to the brachiocephalic veins
What is the ligament that attaches the posterior aspect of the thyroid gland to the cricoid cartilage of the trachea?
The berry ligament
What is the berry ligament also known as?
The posterior suspensory ligament
What nerve travels very close to and can sometimes pierce the berry ligament? It is at risk of damage during a tracheostomy
The recurrent laryngeal nerve
How do the right and left recurrent laryngeal nerves arise? What do they arch round? What provides the thyroid gland with parasympathetic innervation?
The recurrent laryngeal nerves are branches of the vagus nerve
The right arches around the right subclavian artery
The left arches around the arch of the aorta
The recurrent laryngeal nerves provide the thyroid gland w/ parasympathetic innervation
What are the colloid containing sacs in the thyroid known as? What forms the outer circle of the follicle?
The colloid containing sacs are follicles
Follicular cells form the outer circle
What does the colloid contain? What are the cells that secrete calcitonin known as in the thyroid gland?
Colloid contains - tyrosine-containing thyroglobulin
The cells that secrete calcitonin are known as parafolicular cells (C cells)
What is the main fuel for the thyroid gland? Where is this fuel taken up in the thyroid gland?
Iodine
Iodine is taken up by the follicular cells from the blood and transported to the colloid
When the iodine is taken into the colloid, what does it combine with to form what?
Comines with tyrosine residues on the thyrglobulin molecule to form moniodotyroisne (MIT) or di-iodotyroisne(DIT) molecules
How many MITs and DITs are in T3 and T4? Where are T3 and T4 stored and how are they released?
T3 is formed by the coupling of a MIT + DIT
T4 is formed by the coupling of a DIT + DIT
Both T3 and T4 are stored in the colloid thyroglobulin until released via pinocytosis when required
What are the three thyroid hormones?
T3 - tri-iodothyronine
T4 - thyroxine
Calcitonin
What are two of the main drugs used to treat overactive thyroid disease that inhibit the binding of iodine to tyroisne residues on thyroglobulin?
Carbimazole and propylthiouracil
What is th most common, T4 or T3? (accounts for 90% of the hormones secreted) Which is the major biologically active thyroid hormone?
T4 accounts for the vast majority of thyroid hormone however T3 is the major biologically active thyroid hormone
T4 can be converted to T3 by the liver and kidneys
What is the major regulator for the thyroid hormone? Where does the regulator attach and what does it cause?
Major regulator is TSH - thyroid stimulating hormone
TSH acts on a receptor on the follicular cell wall and triggers a release of T3 and T4 into the bloodstream
Why do T3 and T4 require binding to plasma proteins in the blood? What are the three main plasma proteins which they bind to?
because they are hydrophobic / lipophilic
thyroxine binding globulin (TBG ~70%)
thyroxine binding prealbumin (TBPA ~20%)
Albumin (~5%)
Is the bound form of T3 and T4 available to tissues? When measuring the thyroid hormone levels, is it free or bound T3 and 4 that are measured?
Only the unbound/free form is available to tissues
It is the free form of the hormone that are measured in the blood during a blood test
When there is an increase in the total TBG (thyroxine binding globulin), both the total and free T4 increase True or false
False
The total T4 increases but the free T4 remains the same
What are the three ways in which thyroid hormones increase the basal metabolic rate? (think respiration)
Thyroid hormones increase:
Number and size of mitochondria
Increase oxygen use and rates of ATP hydrolysis
Increase synthesis of respiratory chain enzymes
What percentage of temperature regulation is due to thyroid hormone thermogenesis?
30%
How do thyroid hormones increase the levels of blood glucose?
increase blood glucose – due to stimulation of glycogenolysis and gluconeogenesis
increase insulin-dependent glucose uptake into cells
If the foetus does not get an adequate thyroid hormone during pregnancy, what can this cause?
The child can have slow intellectual function
Thyroid hormones increase responsiveness to adrenaline and noradrenaline by increasing the number of receptors. cardiovascular responsiveness also increased due to this effect – increased force and rate of contraction of heart
What drug can be used to treat symptomatic hyperthyroidism due to this increased sympathetic activity?
Give a non-selective beta bblocker eg propranolol
The thyroid hormones give feedback loops to the pituitary and the hypothalamus What hormone is released from the hypothalamus that regulates thyroid function?
Thyrotrophin releasing hormone (TRH), from hypothalamus, stimulates thyroid stimulating hormone (TSH) release from anterior pituitary
TSH = major stimulant to release of T3 and T4 from thyroid gland
What does an exposure to low temperature in children and babies cause in relation to the thyroid gland?
Stimulates TRH release causing release of TSH which causes the release of T3 and T4 to increase thermogenesis
What inhibits T3 and T4 release? (this same thing will cause an increase in glucose)
Stress
Are thyroid hormones highest at night or am? (they follow a circadian rythym)
Thyroid hormones are highest at night
What are the enzymes that are important in thyroid hormone activation and deactivation?, they can convert T4 to T3 and vice versa
Deiodinase enzymes
Which of the three deiodinase enzymes is the major activating deiodinase enzyme by converting T4 to T3?
D2
Type I (Deiodinase 1) is commonly found in the liver and kidney Which deiodinase enzyme is found in fetal tissue and placenta , and the brain? - what is its major function
Deiodinase type 3 - major inactivating deiodinase enzyme (converts T3 to T4)
Is goitre only associated with hyperthyroidism?
No goitre can be associated with hypo or hyper thyroidism
If the deficiency of thyroid hormones is due to TSH or TRH, will there be a goitre?
No
What are symptoms of hypothyroidism?
- Reduced BMR
- Slow pulse rate
- Fatigue,
- lethargy,
- slow response times and mental sluggishness
- Cold-intolerance
- Tendency to put on weight easily
- Can also get scanty periods and
- constipation
What is congenital hypothyroidism also known as? One cause is due to congenital iodine deficiency known eg often caused by insufficient dietary iodine during pregnancy What are the presenting features?
Babies - Cretinism
– dwarfism & limited mental functioning due to deficiency of thyroid hormones present at birth
Goitre poor growth and hair loss are common features
What is a common symptom of hypothyroidism in adults? presents with puffy hands /feet
Myxoedema – puffy face, hands & feet
Does exopthalmos show in every case of hyperthyroidism or only in grave’s? Why does exopthalmos show?
Only shows in grave’s disease the immune system attacks the muscles and fatty tissues around and behind the eye causing the eyes to bulge forwards
What are the symptoms of hyperthyroidism?
Increased BMR
Very fast pulse rate
Increased nervousness and excessively emotional
insomnia
Sweating & heat intolerance
Tendency to lose weight easily
Atrial fibrillation can be a first sign of hyperthyroidism
Graves disease What does it cause? What are the associated antiboides in graves disease and what do they cause?
* it is the number one cause of hyperthyroidism
* it is cause by the IgG antibodies that activate the GPCR, which stimulates follicular hypertrophy and hyperplasia
* thyroid stimulating immunoglobulin (IgG class) (TSI): mimics action of TSH (thyroid-stimulating hormone) and stimulates the release of thyroid hormones, this causes thyroid enlargement and increased production of thyroid hormones, triiodothyronine (T3) and thyroxine (T4)
* there is also thyroid growth-stimulating Ig and TSH-binding inhibitor
- What is average weight of the thyroid in adults?
- What nerve can potentially be damaged during surgery?
- What is the most biologically thyroid active hormone ?
- What is the main regulator of thyroid hormone release?
- What is the main binding globulin for thyroxine?
- Thyroxine has no effect on cardiac muscle t/f
- Thyroxine has no effect on reproductive tissue t/f
- A goitre is only present in hyperthyroidism t/f
- An underactive thyroid causes heat intolerance t/f
25-30g
Recurrent laryngeal nerve
T3
TSH
Thyroxine binding globulin
False False False False
