Thyroid hormones Flashcards
What is the importance of thyroid hormones ( general )
- painting energy homeostasis
- regulating energy expenditure
- regulate cell metabolism activity
what is the location and shape of the thyroid gland
- bowtie shaped gland located in neck
- inferior to larynx and anterior to trachea
what going the two thyroid lobes
section of tissue called isthmus
Explain what the thyroid is made of ( from lobe to colloid )
- thyroid is made of 2 lobes made of lobules which each contain 20-40 follicles , each follicle filled with substance known as colloid
What is the primary constituent of colloid
thyroglobulin : includes residues of amino acid tyrosine
Where does thyroid hormone synthesis take place
in the colloid
What is the C cell ( parafollicular cell )
- produces calcitonin hormone
Is the thyroid highly vascularized
yes
What are the basic ingredients of thyroid hormone synthesis
tyrosine and iodine
- iodine needs to be consumed in our diet
What are the steps of TH synthesis
1- iodine + tyrosine + monoiodotyrosine ( MIT)
2- MIT + iodine + diiodotyrosine ( DIT )
3- DIT + DIT = thyroxine ( T4 ) - TH
4- DIT + MIT + triiodothyronine ( T3 ) - TH
Name the 2 thyroid hormones
thyroxine and triiodothyronine
What is the only tissue in the body that requires iodine
thyroid
How does iodine go to thyroid
- ingested then converted to iodide in stomach
- enters thyroid by co-transportation with sodium
- diffuses into the follicle where it’s turned back to iodine
What is TPO ( thyroidperoxidase )
- enzyme required to attach iodine to tyrosine
What happens when TH is required ( 5 steps )
1- section of colloid is endocytozed into follicular cell
2- endosome binds with lysosome
3- T3 & T4 are removed from thyroglobulin molecule
4- they’r lyophilic so they cross cell membrane and bind to transporters in blood
5- excess MIT & DIT are recycled back into colloid
What is the function of radioactive iodine
- radioactive particle is orally ingested, enter thyroid cell then cell glows or dies
- used diagnostically as tracer or therapeutically
80 -90% of TH released is in what form
T4
Why does T4 need to be converted to T3 and how
- T3 is more biologically active
- combines with greater affinity to nuclear receptors
- T4 needs to be deiodinated to become T3
What converts most T4 to T3
- Type 1 deiodinase
- produced in liver and kidney
Where is most T4 converted to T3
liver and kidney
How are TH transported in plasma , give examples of proteins types
- they’re lypophilic and are transported by thyroid-binding proteins
- thyroid binding globulin , thyroid binding prealbumin, thyroid binding albumin
Are there free T4 in blood
- very small percentage
What will form from conjugation in follicular lumen of DIT ?
a: diiodothyronine b: RT3 c: most potent form of thyroid hormone d: thyroid hormone in circulating majority
d
where are the majority of TH receptors
- receptors in nuclei of cells
- all over body
What is the action of TH in cells ( 3 )
- genomic affects which activates protein synthesis :
- increase metabolic activity
- increase ATP
- increase basal metabolic rate
Describe metabolic affects of TH ( 7 )
- increased oxygen consumption ( due to increased metabolism in cells )
- increased BMR
- calorigenic effect - burning calories
- thermogenic effect - linked to calorigenic effect
- carb metabolism increases
- lipid metabolism - lipolysis
- protein metabolism - synthesis and breakdown
How is carb metabolism affected by TH
it’s increased since TH increases glucose absorption from GIT , glycogenolysis and gluconeogenesis
What is the favourable affect on fat metabolism due to TH
- decreased of cholesterol levels
Affect of TH on Heart
- increases Beta 1 receptors in heart tissue: increase HR, CO, SBP , decrease DBP
- increase blood flow to skin
Affect of TH on Lungs
- increase ventilation rate ( consuming a lot more 02 )
Affect of TH on GIT
- increase appetite, secretion of digestive juices and GIT motility
Affect of TH on Reproductive
- essential for lactation and reproduction
Affect of TH on Musculoskeletal
- promotes body growth and skeleton maturation
- promotes function and development of muscles
Affect of TH on nervous
- promotes neural development in fetus and infants
- promotes neuronal function
- enhances SNS effects ( affects B1 heart receptors )
How are TH levels regulated , what has a positive and negative affect
- Cold has positive affect on hypothalamus for production of TH
- stress had negative affect on hypothalamus for production of TH
- Hypothalamus secretes TRH , acts on pituitary to secrete TSH , acts on thyroid to secrete T3 & T4
TSH action on thyroid gland ( 7 )
1- increases iodine trapping and biding by increasing gene transcription of sodium iodine transporter 2- promotes thyroglobulin synthesis 3- TPO synthesis 4- Stimulates T3 & T4 synthesis 5- Promotes colloid endocytosis 6- increases follicular cells 7- increases blood flow to thyroid
Explain Primary hypothyroidism ( defect, result )
- endocrine gland disorder
- failure of thyroid to respond to TSH
- elevated TSH levels due to reduced negative feedback
- thyroid enlarged = goitre formation
Explain Secondary hypothyroidism ( defect , result )
- pituitary disorder
- deficient TSH production in pituitary
- Thyroid atrophy ( shrinking )
Explain Tertiary thyroid defect
- hypothalamus disorder
- deficient TSH secretion due to deficient TRH secretion
Causes of Primary hypothyroidism ( 4 )
- thyroiditis
- iodine deficiency
- deficiency of synthesis enzymes
- removal or destruction of thyroid gland
What is Hashimoto’s disease
- chronic lymphocytic thyroiditis
- autoimmune antibodies are targeting thyroid glands
What is congenital hypothyroidism and its symptoms
- when thyroid gland does not come to function in baby
- all newborns tested for it
- unhappy baby, difficulty feeding , constipated, lethargic, sleepy
What is the result of not enough exposure of TH in infancy and into adulthood
- cretinism
- short individual , underdeveloped , abnormal face
- brain , bone and skeletal muscle development is impaired
What happens if non working thyroid gland is diagnosed
- thyroid hormones replacements are given
What are the causes of hyperthyroidism ( 3)
1- grave’s disease
2- thyroid adenoma
3- TSH secreting adenoma
Explain Grave’s disease ( defect, result )
- autoantibodies stimulate thyroid gland
- bind to same receptors that TSH binds to
- result in excess T3 & T4 production
How do you diagnose grave’s disease , symptoms
- low TSH levels
- high thyroid hormone levels
- goitre ( enlarged thyroid )
- exophthalmos ( protruding eyes )
- upper eyelid retraction
What causes exophthalmos in grave’s disease
- eyes protrude due to deposition of mucopolysaccharide behind eyes and immune cell infiltration behind eye
Name some symptoms of hyperthyroidism
- hyperactivity
- heat intolerance
- palpitations
- fatigue
- diarrhea
- irregular periods
- big appetite
- weight loss
Name some symptoms of hypothyroidism
- mental sluggishness
- cold intolerance e
- dyspnea
- fatigue
- constipation
- weight gain
- poor appetite
Name some signs of hyperthyroidism
- tachycardia
- tremor
- goitre
- warm skin
- exophthalmos
- proximal muscle weakness
Name some signs of hypothyroidism
- bradycardia
- goitre
- dry skin
- carpal tunnel
- puffy face
- peripheral edema
What is the affect on T4, TSH , TRH with primary hypothyroidism
- decrease T4
- Increase TSH , TRH
What is the affect on T4, TSH , TRH with secondary hypothyroidism
- Decrease T4 , TSH
- increase TRH
What is the affect on T4, TSH , TRH with tertiary hypothyroidism
- decrease T4, TSH , TRH
What is the affect on T4, TSH , TRH with Grave’s disease
- decrease TSH
- Increase T4 , TRH
How does hypothyroidism result in thyroid hypertrophy
- low T3 & T4 = reduced negative feedback = more TSH and TRH = increase in thyroid gland size due to increase in trying produce T3 & T4
