Thyroid Physiology Flashcards
What all does the thyroid gland tissue secrete?
Thyroxine (T4)
Tri-iodothyronine (T3)
Calcitonin
What do the parathyroid glands secrete
Parathyroid hormone (PTH)
What cells secrete calcitonin
Parafollicular C cells
Describe what can be seen under the microscope for thyroid tissue
Follicular cells
Colloid - tyrosine containing thyroglobulin filled spheres enclosed by follicular cells
Parafollicular C cells
What does calcitonin play a minor role in
The regulation of Calcium
What is the main substance in the colloid
Thyroglobulin
What do tyrosine units do?
Convert T3 to T4
How is T3 formed
The coupling of MIT and DIT
How is T4 formed
coupling of 2 DITs
Where is T3 and T4 found
In the colloid thyroglobulin till required
Where is iodine taken up
follicle cells
How is MIT and DIT formed
Iodine attaches to tyrosine residues on thyroglobulin
What do T3 and T4 bind to and why
plasma proteins as they hydrophobic / lipophilic
When are T3 and T4 not biologically active
When they are bound - they need to be released by the carrier as they are required into the tissues
Is T3 more active than T4 or other way about
T3 is more active than T4
How much of thyroid hormones that are secreted are T4
90%
What is the more potent Thyroid hormone
T3 - it is around 4 times more potent
What converts T4 to T3
Liver and kidneys
What do thyroid hormones do to the basal metabolic rate
Increase it:
increase number and size of mitochondria
Increase oxygen use and rates of ATP hydrolysis
Increase synthesis of respiratory chain enzymes
What do thyroid hormones do to thermogenesis
Increase it
around 30 % temperature regulation due to thyroid hormone thermogenesis
Why is there an increase in blood glucose because of thyroid hormones
Due to stimulation of glycogenolysis and gluconeogenesis as well as an increase of insulin-dependent glucose uptake into cells
What do thyroid hormones do to lipid metabolism
Mobilise fats from adipose tissue and increase fatty acid oxidation in tissues
What do thyroid hormones do to protein metabolism
Increase protein synthesis
Why is the thyroid hormone important in growth and development
It is important in the production and secretion of growth hormone
Development of foetal and neonatal brain
Normal central nervous activity is controlled by this
How do thyroid hormones have a permissive sympathomimetic action
They increase responsiveness to adrenaline and sympathetic NS neurotransmitter, noradrenaline, by increasing numbers of receptors
What should be used to treat initial stages of hyperthyroidism
Beta blockers such as propranolol
How are the release of TRH and TSH controlled
Through negative feedback of T3 and T4
What effect does stress have on the thyroid hormone
It inhibits TRH and TSH release
How do low temperatures affect thyroid hormone regulation
Stimulation of TRH release which stimulates TSH release and so increases T3 and T4 release from thyroid
Describe the levels of thyroid hormones throughout the day
Circadian rhythm
Highest late at night and lowest first thing in the morning
What are some symptoms of a deficient thyroid hormone (hypothyroidism)
Reduced BMR Slow pulse rate Fatigue, lethargy, slow response times and mental sluggishness cold intolerance tendency to put on weight easily
What is another symptom for adults with hypothyroidism
Myxoedema (puffy face, hands and feet(
What is another symptom for babies with hypothyroidism
Cretinism (dwarfism and limited mental functioning due to deficiency of thyroid hormones present at birth
What causes exophthalmos
Water retaining carbohydrate build up behind the eyes
What is the main binding globulin for thyroxine
TBG
