Thyroid Physiology Flashcards
What is secreted from thyroid gland tissue?
Thyroxin
Tri-iodothyronine
Calcitonin
What is secreted from the parathyroid glands?
Parathyroid hormone
What ion is the parathyroid gland in control of?
Calcium
Where does the thyroid sit?
Between the 5th cervical and 1st thoracic vertebrae
What is the blood supply to the thyroid gland?
Superior and inferior thyroid arteries from the external caroti
Thyroidea ima in some people
What nerve runs alongside the thyroid gland?
Recurrent laryngeal nerve
What drains the thyroid gland?
Superior/ middle thyroid vein which drain into internal jugular
Inferior thyroid that drains into braciocephalic vein
What are the thyroid producing cells?
Follicular cells
What is a colloid?
Tyrosine containing thyroglobulin filled spheres enclosed by follicular cells
What cell produces calcitonin?
Parafollicular C cells
What is the action of calcitonin?
Opposite of PTH - it reduces calcium levels
What ion is needed to make thyroid hormones?
Iodine
In what cell is iodine found in inside follicualr cells?
Pinocytosis
What is the building blocks of thyroid hormones?
Iodine attached to tyrosine residuces on thyroglobulin
Which hormone is more potent - T3 or T4?
T3 is 4x more potent
Where is T4 converted to T3?
In the liver and kidneys - but can be converted in any cell in the body
Which hormone is the major biologically active thyroid hormone?
T3
What does TSH do?
Stimulates the follicualr cell to secrete thyriod hormones
How do T3 and T4 travel around the blood?
Bound to plasma proteins due to the hydoophobic nature
What are the main plasma proteins that thyroid hormones bind to?
Thyroxin binding globulin
Thyroxine binding prealbumin
Albumin
When will the thyroid hormones be biologically active?
When unbound and free in the plasma
How is T4 converted to T3?
Removal of an iodine ion
What receptors do thyroid hormones bind to at a cellular level?
Hybrid nuclear receptors
What do thyroid blood tests measure?
The free levels of T4 and T3
What can increase the total T4 but not free T4?
Pregnancy
OCP
What effects does thyroxine have?
Metabolism Growth Development Reproduction Behaviour
What will an overactive thyroid do to the skin?
Increased sweat
What will an overactive thyroid have to the heart?
Increased HR
Palpitations
What can increased thyroxine do in gynae?
Reduced periods
For what organ is thyroxine particularly important for in a developing foetus?
Brain
What do thyroid hormones do the basal metabolic rate?
Increased basal metabolic rate
Increase number and size of mitochondria
Increase oxgen use and rates of ATP hydrolysis
Increased synthesis of resp chain enzymes
What effect do thyroid hormones have on thermogenesis?
Increased
What effect do thyroid hormones have on carbohydrate metabolism?
Increase blood glucose
Increase insulin-dependent glucose uptake into cells
What effect do thyroid hormones have on lipid metabolism?
Mobilise fats from adipose tissue
Increase fatty acid oxidation in tissues
What effect do thyroid hormones have on protein metabolism?
Increased protein synthesis
What effect do thyroid hormones have on growth?
Growth hormone releasing hormone (GHRH) production and secretion requires thyroid hormones
Glucocorticoid induced GHRH release also dependent on thyroid hormones
What effect do thyroid hormones have on developement of foetal and neonatal brain?
Myeliongenesis and axonal growth require thyroid hormones
Particularly imporant in 1st trimester as foetus doesn’t have thyroid gland yet
What effect do thyroid hormones have on normal central nervous system activity?
Hypothyrodism - slow intellectual function
Hyperthyrodism - nervousness, hyperkinesis, emotional lability
What neurotransmitters are affected by thyroid hormones?
Thyroid hormones increase the responsivness to adrenaline and sympathetic neurotransmitter noradrenaline by increasing the number of receptors
What happens in the heart in increased thyroid production?
Increased force and rate of contraction
What drug can be used to treat the inital symptoms of hyperthyrodism?
Beta blockers such as propanolol
What is the effect of stress on thyroid hormone regulation?
Inhibitis TRH and TSH release
How does the circadian rhythm affect thyroid hormone regulation?
Highest late at night and lowest in the morning
How do low temps affect thyroid hormone regulation?
In babies and young children, expsoure to cold environments stimualtes TRH release, stimulating TSH release increasing T3 and T4 release
What is the function of throtrophin releaseing hormone (TRH)?
Formed in the hypothalamus and stimulates TSH release from the anterior pituitary
What type of feedback is utilised in thyroid hormone regulation?
Negative feedback control of release of TRH and TSH
What is the function of deiodinase enzymes?
Activate and deactivate thyroid hormones via the addition or removal of an iodine atom in the outer ring
What is the most important deiodinase enzyme?
Type 2 - D2
What does D2 do?
Converts T4 to T3
Where it D1 found?
Liver and kidneys
Where is D2 found?
Heart and skeletal muscle, CNS, fat, thyroid and pituitary
Where is D3 found?
Fetal tissue and placenta, brain (except pituitary)
What can cause hypothyrodism?
Primary gland failure - may be associated with goitre - autoimmune disease
Secondary to TRH or TSH (no goitre)
Lack of iodine in diet
What are the symptoms of hypothyrodism?
Reduced BMR Slow pulse rate Fatigue Cold-intolerance Tendency to put on weight easily
What is myxoedema?
Puffy face, hands and feet
What is cretinism?
Dwarfism and limited mental function due to deficiency of thyroid hormones at birth
What is the most common cause for hypothyrodism?
Autoimmune disease
What is the most common reason for hyperthyrodism?
Graves disease - autoimmune disease where thyroid stimulating immunoglobulin acts like TSH
What is exopthalmos?
Bulging eyes due to water retaining carbohydrate buld up behind eyes
What are the common symptoms of hyperthyrodism?
Increased BMR Vert fast HR Increased nervousness and excessively emotional Insomnia Sweating and heat intolerance Tendency to lose weight easily
What is the average weight of the thyroid gland in adults?
25-30g
What is the main regulator of thyroid hormone release?
TSH
What is the main binding globulin for thyroxine?
TBG - thyroxine binding globulin
What effects can thyroxine have on the heart?
Cardiomyopathy
AF