thyroid gland Flashcards
(35 cards)
Where is the thyroid gland
what is the structure of the thyroid gland?
- adheres to the trachea
- 2 large asymmetrical lobes connected by isthmus
- 4 parathyroid glands on back
- plasma calcium control
what is the thyroid gland controlled by?
regulated by the hypothalamus and pituitary gland
blood supply to the thyroid gland
rich blood supply
- Superior and inferior thyroid arteries
- External carotid branches
histology of thyroid gland
structrue
Functional unit = follicle
- 1000s in each gland
- Each follicle consists of layer of follicular cells (simple cuboidal epithelial) surrounding a colloid-filled cavity
- Follicular cells surrounding colloid
- C cells - involved in calcium regulation
what hormones does the thyroid secrete
where are they produced
Triiodothyronine (T3) and thyroxine (T4)
both produced within the follicles
what is the difference between T4 and T3
- T4 contain four atoms of iodide per molecule
- T3 contains 3 atoms of iodide per molecule
- T3 has much higher biological activity
- yet more T4 is secreted by the thyroid
Why is less than 0.5% of T3 and T4 in ‘free form’??
they are bound to plasma proteins: thyroxine-binding globuli and albumin
- prolongs half life
- keeps them in circulation
- prevents them from being exctreded immediately
how are T3 and T4 synthesised?
requires tyrosines and iodine
- tyrosines
- provided by thyroglobulin
- makes up colloid
- secreted by follicular cells into lumen of follicle as colloid
- provided by thyroglobulin
- iodine
- pumped into follicular cells against concentration gradient
- iodine binds to thyroglobulin
- becomes iodinated thyroglobulin
- lysosomal enzymes cuts up the molecule
- ends up with iodinated tyrosine - T3 and T4
what are monoiodotyrosine (MIT) and diiodotyrosine (DIT)
- Two molecules of iodine bind to 1 tyrosine
- Diiodotyrosine
- Opposite -
- Monoiodotyrosine
how do monoiodotyrosine (MIT) and diiodotyrosine (DIT) make up T3 and T4
T3 =
- 1 monoiodotyrosine (MIT) and 1 diiodotyrosine (DIT)
T4 =
- 2 diiodotyrosine (DIT)
advantages of gaining thyroid hormone from thyroglobulin and iodine
- The thyroid gland is capable of storing many weeks worth of thyroid hormone within the colloid
- (coupled to thyroglobulin).
- Iodine can be stored in the form of iodide also
- If no iodine is available for this period, thyroid hormone secretion will be maintained.
What must occur for thyroid hormone to be made functional?
T4 must be converted into T3
peripheral conversion
done in liver and kidneys after being sent out into circulation
what is given to hypothyroid individuals and why?
T4
is then converted into T3
T3 is not given as it is not stable enough to be given as a tablet
effects of thyroid hormone
- Calorigenesis
- Growth & maturation rate
- C.N.S. Development & function
- CHO, fat & protein metabolism
- Muscle metabolism
- Electrolyte balance
- Vitamin metabolism
- Cardiovascular system
- Hematopoietic system
- Gastrointestinal system
- Pregnancy
principal actions of thyroid hormones
Metabolism:
- Increases mobilization and utilisation of glucose, fat, protein
Heat production (calorigenic effect)
- Important in temperature regulation and adaptation to cold environments
- Often those with hypothyroidism
- Feel cold - cannot regulate temperature well
Growth & Development
- Essential for normal growth of tissues, including CNS
- Synergy with growth hormone
- Thyroid hormone deficiencies can result in mental impairment and short stature
Cardiovascular function
- Increased cardiac output, heart rate and contractility
- Increases ventilation rateHypothyroid individuals
- Low heartrate
- Hyperthyroid individuals
- Higher heartrate
draw negative feedback loop of thyroid hormones
TRH - thyrotropin releasing hormone
TSH - thyroid stimulating hormone
How does TSH affect thyroid function
- including promoting the release of thyroid hormones into blood stream,
- increasing the activity of the iodide pump and iodination of tyrosine
- to increase production of thyroid hormones
symptoms of hypothyroidism
- Dry, cold skin
- Sensitivity to cold
- Weight gain despite loss of appetite
- Impaired memory
- Mental dullness
- Lethargy
- Myxedema:
- cutaneous and dermal oedema secondary to increased deposition of connective tissue components
- Excess fluid tissue under eye
- Excess GAG deposition
how could you tell from a clinical exam that someone has hypothyroidism
- Reduced metabolic rate
- Slow pulse
- Reduced cardiac output
how would you diagnose someone who has hypothyroidism
Low plasma levels of ‘free’ T3 and T4
how would you treat hypothyroidism
Thyroxine
- Dose determined by TSH monitoring
- Individuals with hypothyroidism have increased TSH levels
- Excess TSH trying to stimulate thyroid gland
- TSH levels should decrease after supplementations of thyroxine
causes of hypothyroidism
- Iodine deficiency
- Endemic goitre
- Autoimmune disease
- Hashimoto’s thyroiditis
- Others:
- Congenital
- Post radiation / surgery
- Medication
- stress
what is endemic goitre?
what is the treatment?
- iodine deficiency
- insufficient dietary iodine
- insufficient T3 and T4
- Reduction in negative feedback of TSH by the pituitary
- Abnormally high TSH
- Abnormal growth of the thyroid due to the trophic effects of TSH
- Enlarged thyroid gland
- Treatment
- Iodine supplements
- Thyroxine to reduced TSH levels
what is hashimoto’s disease
- Most common cause of hypothyroidism
- Autoimmune disease
- Antibodies against TSH receptor (in the thyroid gland)
- Prevents TSH binding its receptor
- Prevents stimulation to T3 & T4
- Antibodies also against thyroid peroxidase & thyroglobulin
- Leads to thyroid gland destruction
- TSH is being produced normally
- Receptor is attacked so cannot bind
- Less T3 & T4 production
