thyroid gland physiology Flashcards
what 2 molecules does the thyroid gland produce?
- thyroxine -T4 (as it contains 4 iodines)
- Triiodothyronine - T3 (three iodines) - most T4 is converted to T3 so this is the major thryoid hormone
where is the thyroid gland situated?
situated in the midline of the anterior neck
what structure seperates the right and left lobe of thyroid gland?
the isthmus
what does the thyroid gland contain?
- there are numerois follicles within the thyroid gland
- each of the follicles are composed of a sphere of epithelial cells (follicular cells)
- these cells surround a core containing a protein rich material called a colloid
what is the function of these follicular cells?
they **synthesise and secrete thyroid hormones **in response to the anterior pituitary thyroid stimulating hormone (TSH)
other than follicular cells, what is the second type of endocrine cell found in the thyroid gland?
- parafollicular cells
what is the** function of parafollicular cells**?
- they secrete a peptide hormone called calcitonin which plays a role in Ca2+ homeostasis
what does the synthesis of the thyroid hormones require?
iodine
Describe the steps synthesis of the thyroid hormones
- Thyroid hormone synthesis requires iodine ( supplied from blood)
- iodide uptake from blood is performed by follicular cells in thyroid through secondary active transport - coupled to Na+ - iodide trapping
- oxidation of I- to I+ (iodide - iodine) & reacts with tyrosine residues of thyroglobulin to form monoiodotyrosine and diiodotyrosine
- formation of T3 & T4 from MIT/DIT
where are the thyroid hormones stored?
in the colloid
what is T3 and T4’s mechanism of action in target cells?
- T3 & T4 enter target cells where T4 is deionated to T3
- T3 is transported into nucleus and binds to thyroid hormone receptor ( TR)
- the hormone receptor complex binds to the hormone response element HRE and **alters the rate of gene transcription **
what hormone is responsible for oxidising iodides and attaching them to tyrosines on thryglobulin ?
thyroid peroxidase
what are the physiological functions of T3/T4?
- regulate basal metabolic rate (the rate at which the body uses energy while at rest to maintain vital functions)
- important for CNS development
- important for growth
how are T3/T4 carried/transported in the plasma?
bound to plasma proteins
what are the 3 major plasma proteins that carry T3 or T4?
- thyroxine binding protein (TBG)
- transthyretin
- albumin
what enzyme converts T4 into T3?
- 5’ monodeiodinase
what enzyme inactivates T3 in liver ?
5 monodeiodinase
note no ‘
what does the basal metabolic rate depend on?
- sex
- body surface area
- activity level
- body composition
what are the** effects** of the thyroid hormones on cellular respiration and metabolism ?
thyroid hormones increase the
* size and No. of mitochondria
* oxygen consumption
* ATP production
* protein and lipid turnover
* **increases basal metabolic rate **
why is thyroid hormone essential for normal growth?
- protein synthesis
- increases energy production and substrate availability
why is thyroid hormone essential for CNS development?
- promotes nerve growth
- growth of dendrite cells
- myelin formation
what is cretinism?
*** untreated congential hypothyroidism **(at birth) - ie partial or complete loss of thyroid gland function
* causes impaired CNS development & intellectual/ developmental disability
How does thyroid hormone affect the heart? (ie HR and cardiac output)
- thyroid hormone stimulates the release of metabolic endproducts
- this causes local vasodilatation which reduces systemic resistance
- then the blood pressure also decreases
- TH causes increase in HR and cardiac output & contractility of the heart
what is the hypothalamus-pituitary-thyroid axis?
- **thyroid function is controlled by the hypothalamus and pituitary gland **
- hypothalamus - TRH regulates TSH secretion in anterior lobe
- anterior pituitary lobe -secretes **TSH **which increases the production of T3 and T4
what effect does T3 and T4 have on TSH & TRH production?
negative feedback
other than increasing the production of T3 and T4, what other effects does TSH have?
- increases protein synthesis in follicular epithelial cells
- maintenance and growth of gland
what can happen if the thyroid gland cells are exposed to greater TSH concentrations & what condition can this occur in?
- they will undergo** hypertrophy **- ie they will **increase in size **
- goiter (enlarged thyroid gland)
- this can occur in hypothyroidism - a low amount of T3/T4 results in increases in TSH - goitre
what is** hypothyroidism**?
- a condition in which the plasma conc of thryoid hormones is below normal
- most are due to damage or loss of function of thyroid tissue or inadequate iodine consumption
Describe hypothyroidism
what is it? what occurs the the HPT axis as a result?
- the synthesis of thyroid hormone is compromised
- this releases the hypothalamus and the pituitary gland from the negative feedback inhibition
- this leads to an increase in TRH conc and also TSH conc
- TSH will cause goitre of thyroid gland
what are the common symptoms of hypothyroidism?
- goitre (in adults)
- decrease in appetite - decrease in BMR
- decrease in heart rate and cardiac output - as less TH will stimulate increases in HR and CO
- tiredness - low BMR
- weight gain - low BMR
- cretinsim in children
what is primary hypothyroidism?
- abnormality of thyroid itself
- may be due to** iodine deficiency,genetic causes, autoimmune (antibodies against TSH receptor)
what is secondary hypothyroidism?
a problem with the hypothalamus or pituitary gland
How is **primary hypothyroidism diagnosed **? - also include what might be present if caused by autoimmune disease
NB - exam?
- decreased T3 & T4
- increased TSH
- if autoimmune - presence anti-thyroid antibody
what is the autoimmune disorder associated with primary hypothyroidism called & what does it involve?
- Hashimoto’s Thyroiditis
- antibodies against TSH receptor which act as TSH antagonists
how is primary hypothyroidism treated?
- replacement therapy with levothyroxine
- or dietary iodine supplement
what is myxedema?
puffiness in the face - increased water retention within the connective tissue
what is hyperthyroidism?
when there is an excess of thyroid hormone in the plasma
what are some of the** causes of primary hyperthyroidism**? (prob with thyroid itself)
- autoimmune disorder - Graves disease
- thyroid tumour
what is graves disease?
- autoimmune disorder
- production of antibodies that bind to and activate the TSH receptors on thyroid gland cells - which leads to chronic overstimulation of the growth and activity of the thyroid gland - may lead to goitre
what are the symptoms of hyperthyroidism?
- nervousness
- increase in heart rate - may cause arrythmias - TH increases HR etc
- goitre - due to overstimulation of gland - hypertrophy
- myxedema - bulging eyes
- increase in BMR- therefore increase in body temp
- weight loss - high BMR
How ia hyperthyroidism diagnosed?
- increased T3/T4 production
- decreased TSH( in primary and elevated in secondary)
- if autoimmune - presence of antibodies to TSH
how can hyperthyroidism be treated?
- with agents that interfere with the production of T3 or T4
- destruction of thyroid gland eg thryoidectomy - surgical removal of some of gland
what is** thyrotosis crisis**?
- a life threatening complication associated with untreated or undertreated hyperthyroidism
- symptoms include tachycardia, arrhythmias, dehydration etc
what medication can be used to** treat thyrotoxic crisis**?
- beta blockers to reduce sympathetic activation
- glucocorticoids - to reduce T4 to T3 conversion
