Flashcards in Thyroid Gland Deck (25):
Location of gland
Anterior to upper trachea and lower lanynx
Structure of gland
Type of cells found in it
Butterfly shaped with two lobes connected by an isthmus
2-3 cm across
How are cells arranged in thyroid?
Spherical follicles surround central space and are lined ewith epithelial cells
Hormones produced in thyroid?
How are T3 and T4 synthesised?
-- transport of iodide into the epithelial cells against a concentration gradient.
-- synthesis of a tyrosine rich protein (thyroglobulin) in the epithelial cells.
-- secretion (exocytosis) of thyroglobulin into the lumen of the follicle
-- oxidation of iodide to produce an iodinating species.
-- iodination of the side chains of tyrosine residues in thyroglobulin to form MIT (mono-
iodotyrosine) and DIT (di-iodotyrosine).
-- coupling of DIT with MIT or DIT to form T3 & T4 respectively within the thyroglobulin. T3 & T4 residues are produced in the ratio of ~1:10
Storage of T3 and T4?
Lumen of follicles in vesicles
How is release of T3 and T4 controlled?
TRH from hypothalamus which stimulates ...
TSH from pituitary which stimulates ...
T3 and T4 from thyroid
What increased release of TRH?
How does it travel to stimulate release of TSH?
Stress and decreasing temperature
Hypothalamic pituitary portal system
Where is TSH released from?
When is it usually released?
Thyrotrophs in pituitary gland
Diurnal rhythm -- increase release at night and decreased in morning
What does TSH do?
Bind to receptors on follicular cells stimulating synthesis and secretion of T3 and T4.
Also has trophic effect on follicular cells which can lead to goitre
How are thryroid hormones transported in the blood?
Transported in the blood bound to proteins (thyronine binding globulin, pre-albumin and albumin)
--- only small amount is free and therefore able to exert its effects on the body
Describe changes that take place in pregnancy to thyroid hormone production
Oestrogens increase the synthesis of TBG during pregnancy and this produces a fall in the amount of T3 & T4 in the circulation as more is bound. The fall in free T3 &T4 removes the inhibitory feedback on the pituitary and hypothalamus. More TRH and TSH are produced and the thyroid gland secretes more T3 & T4. As a result the amount of free T3 & T4 returns to normal but the total amount in the blood is increased.
What are the physiological actions of T3 and T4?
1. Increased metabolic activity -- increased glucose uptake & metabolism, increased protein metabolism. Lead to increased BMR, O2 consumption & heat production
2. Normal growth and development -- bone mineralisation
3. CNS -- development of nerve cells, hyperplasia of cortical neurons and myelination of nerve fibres.
4. Stimulate hormone and neurotransmitter receptor synthesis in a variety of tissues (eg tachycardia in heart muscle, increased motility in GI tract). This increases responsiveness in tissues
5. Role in LH and FSH actions -- ovulation doesnt occur in absence of
What is cretinism?
In the absence of thyroid hormones from birth to puberty the child remains mentally and physically retarded (cretinism).
Describe how T3 and T4 have their effect on cells
T3 & T4 cross the plasma membrane of target cells and interact with specific high affinity receptors located in the nucleus and possibly mitochondria.
Binding of T3 to the hormone-binding domain is thought to produce a conformational change in the receptor that unmasks the DNA-binding domain.
Interaction with DNA binding domain increases the rate of transcription of specific genes that are then translated into protein.
Due to increased protein production, there is an increased demand for energy.
How can T4 be converted into T3?
T4 can be converted to T3 in tissues by removal of the 5'-iodide.
How can reverse T3 be produced?
Removal of the 3'-iodide produces inactive reverse T3 (rT3). rT3 can bind to thyroid hormone receptors without stimulating them, but it blocks the effect of T3.
Too much thyroid hormones
Too little thyroid hormones
Describe hashimotos disease
most common form of hypothyroidism
It is an autoimmune disease caused by:
-- destruction of the thyroid follicles (no synthesis of T3 and T4)
-- production of an antibody that blocks the TSH receptor on follicle cells preventing them from responding to TSH (no T3 and T4 production)
How is hashimotos treated?
Oral thyroxine (T4)
Describe Graves disease
most common form of hyperthyroidism
autoimmune disease in which antibodies are produced that stimulate the TSH receptors on follicle cells resulting in increased production and release of T3 & T4.
How is graves disease treated?
Treated with carbimazole
-- drug that inhibits the incorporation of iodine into thyroglobulin
-- Inhibits the enzyme thyroid peroxidase and therefore prevents coupling and iodination
Hypothyroidism symptoms and why they occur
-- Cold intolerance (reduced BMR)
-- Weight gain (reduced BMR)
-- Tiredness and lethargy (reduced nutrient uptake)
-- Bradycardia (reduces responsiveness to catecholamines and reduced synthesis of heart muscle proteins)
-- Neuromuscular system - weakness, muscle cramps and cerebellar ataxia (clumsiness of
-- Skin dry and flaky (reduced protein synthesis)
-- Alopecia (reduced protein synthesis)
-- Voice is deep and husky
-- Constipation (reduced responsiveness of GI tract)