Thyroid 1 Flashcards
What does the thyroid gland secrete?
Thyroxine (T4), Tri-iodothyronine (T3), Calcitonin
What do the parathyroid glands secrete?
PTH
What structure makes up the thyroid gland?
Follicles
What makes up follicles?
Follicular cells and inner colloid
What is the colloid?
Tyrosine containing thyroglobulin filled spheres enclosed by follicular cells
What do parafollicular C cells secrete?
Calcitonin
What happens in iodide sequestration?
Inorganic iodide is trapped by the gland by an enzyme dependent system, oxidised and incorporatd into the glyco protein thyroglobulinto form mono and diiodotyrosine, thyroid hormone precursors.
What coupling occurs after the production of MIT and DIT, and where are they stored?
MIT+DIT (T3), AND 2 DITs(T4) which are stored in colloid thyroglobulin till required
How much of the thyroid hormones secreted is T4?
~90%
How many times more potent is T3 than T4?
Roughly 4x
What organs convert T4 to T3?
Liver, kidney, (and muscle)
How do T3 and T4 move from the colloid to follicular cell?
Pinocytosis
Why do T3 and T4 bind to plasma proteins when released from follicular cells?
As they are hydrophobic and lipophilic
What plasma proteins do T3 and T4 bind to in the blood stream?
Thyroxine binding globulin (around 70%), Thyroxine binding prealbumin (TBPA around 20%), albumin (around 5%)
In what form at T3 and T4 biologically active?
Unbound
How much less avidly is T3 bound by TBG and TTR than T4?
10-20x less avidly by TBG, and not significantly at all by TTR
What does an increased /decreased TBG level cause?
Increased/decreased total T4 but not free T4
What are some causes of an increased TBG?
Pregnancy, newborn state, oestrogen sources, Tamoxifen, Hep A
What are some causes of decreased TBG?
Androgens, large doses glucocorticoids, Cushings S, active acromegaly, severe systemic illness
What do carbimazole and propylthiouracil inhibit in the treatment of hyperthyroidism?
Iodine attachment to tyrosine residues to form MIT and DIT
What do thyroid hormones do the basal metabolic rate and thermogenesis?
Increase them
What do thyroid hormones do to carbohydrate metabolism?
Increase blood glucose, and increase insulin-dependent glucose uptake into cells
What do thyroid hormones do to lipid metabolism?
Mobilise fats from adipose tissue, increase fatty acid oxidation in tissues
What do thyroid hormones do to protein metabolism?
Increase protein synthesis
Why are thyroid hormones important in growth?
GHRH production and secretion, glucocorticoid-induced GHRH release and GH/somatomedins require thyroid hormones or the presence of them for activity
How do thyroid hormones effect development of foetal and neonatal brain?
Myelinogenesis & axonal growth require thyroid hormones
What are the effects of thyroid hormone on the sympathetic nervous system?
Increased catecholamine sensitivity and B-adrenergic receptor numbers in heart, skeletal muscle, adipose cells and lymphocytes, and decrease cardiac alpha-adrenergic receptors
What are the effects of thyroid hormone on the CVS?
Increases HR and CO
What stimulates TSH release from anterior pituitary?
Thyrotrophin releasing hormone (TRH) from hypothalamus
What do low temperatures do to thyroid hormone regulation?
In babies/young children, cold environments stimulate TRH release, stimulates TSH release so increases T3 and T4 release
What does stress do in thyroid hormone regulation?
Inhibits TRH and TSH
What is the circadian rhythm in thyroid hormone production?
Highest late at night, lowest in morning
What subfamily of enxzymes are important in the activation/deactivation of thyroid hormone at tissue level?
Delodinase enzymes (Type 1,2,3)
How do delodinase enzymes make thyroid hormone inactive?
Addition or removal of an iodine atom in outer ring
What delodinase enzyme activates T4 to T3 in tissues?
D2
Where is D1 found?
Commonly in liver and kidney
Where is D2 found?
Heart, skeletal muscle, CNS, fat, thyroid, pituitary
Where is D3 found?
Fetal tissue, placenta, brain (except pituitary)
What are the most common types of thyroid cancer?
Papillary (76%), follicular (17%)
What hormone drives DTC?
TSH
iS DTC more commen in men or women?
Women
What are the differentiated thyroid cancers?
Papillary and follicular
How does papillary thyroid cancer spread and where to?
Via lymphatics-local spread. Haematogenous spread to lungs, bone, liver and brain
What thyroid condition is papillary thyroid cancer associated with?
Hashimoto’s thyroiditis
What is the prognosis of papillary thyroid cancer?
Good-better if young and female
How does follicular carcinoma tend to spread?
Haematogenously to lung and bone.
What is the prognosis of follicular carcinoma?
Good
What investigations are required in DTC?
After initial thyroid tests, US guided FNA, can involve excision biopsy of LN
What is the usual management of DTC?
Surgery- lobectomy with isthmusectomy, sub-total or total thyroidectomy
