THyroid Gland Flashcards
What does the thyroid produce and secrete via what cells
produces the prohormone Tetraiodothyronine (T4) and the active hormone Triiodothyronine
they are synthesized by the Follicular epithelial cells
WHat doe the C cells secret
Parafollicular cells that secret calcitonin
what is the name of the follicular lumen and what is stored in them
Colloid
this is where the newly synthesized hormones attach to the thyroglobulin
Iodine is also stored here as iodinated thyroglobulin
what makes up T4 structurally
2 diiodotyrosine (DIT)
what makes up T3 structurally
1 monoiodotyrosine (MIT) 1 diiodotyrosine (DIT)
how much more T4 is produced then T3
10 times because T4 is the major secretory part
how is T3 produced in the periphery
T3 is produced by deiodinase via T4 or thyroxine
80 to 90 percent of T3 is produced in the periphery
What are some clinical states in which there is reduction in the conversion of T4 to T3
Fasting
Medical and surgical stress
Catabolic disease
What is the process of the synthesis of Thyroid hormones
Iodide enters the Follicular epithelial cell via the NIS from the bloodstream
THyroglobulin produced by the ER goes into the lumen there Iodine binds to the TG via peroxidase
this goes until T4, T3, MIT, and DIT are bound to the TG and stored in the Colloid
then when needed the cell pinocytosis the TG containing all the items
then proteases cleave off T3 and T4 to go and circulate
What does a deficiency in Deiodinase mimic
Dietary insufficiency of I- (iodide)
What T3 or T4 is favored if there is a restriction in Iodide
T3 is favored when the availabillity of Iodide is restricted
what happens if their is a mutation in Pendrin
Pendrin is a chloride/iodide pump into the colloid
if mutated can lead to affects in the cochlea leading to sensorineural hearing loss
this is called Pendred syndrome also usually have hypothyroidism and goiter
What can PTU be used for
To treat hyperthyroidism by blocking Peroxidase
what is the Wolff-Chaikoff effect
when there are high levels of Iodide it will inhibit the peroxidase and the process of organification and block the synthesis of Thyroid hormones
How can the activity of thyroid gland be assesed
via radioactive iodine uptake
During radioactive Iodide assesment, how does an individual with graves disease present
Have a extreme stimulation and uptake of the Iodide followed by a high turnover of the Iodide
How do thyroid hormones circulate the bloodstream
99 percennt are bound to a plasma protein ( THyroxine binding globulin, if not by that will get picked up by TTR or albumin
one percent is free
What is the half life of circulating T4 and T3
T4 half life is 6 days
T3 half life is 1 day
WHat is the purpose of the T3 Resin uptake test
Determines how much T3, TBG and T4 are circulating in the blood
the resin will bind unbound labeled T3 that did not bind to the unbound TBG
from there you can determine the amount of TBG, T4 and T3 resin bound levels
WHat are the levels of T4 and T3 resin uptake in: Hyperthyroidism
high T4
High T3 resin uptake
WHat are the levels of T4 and T3 resin uptake in: Hypothyroidism
Decrease T4
Decrease T3 resin uptake
WHat are the levels of T4 and T3 resin uptake in: high TBG
high T4
low T3 resin uptake
WHat are the levels of T4 and T3 resin uptake in: LOw TBG
low T4
high T3 resin uptake
WHat are the levels of T4 and T3 resin uptake in: Hepatic failure and how does that affect the synthesis and secretion of thyroid hormones
low TBG
High T3 resin uptake
therefore high levels of free T3 and T4 and serve as negative feedback that inhibits the synthesis of T3 and T4
WHat are the levels of T4 and T3 resin uptake in PRegnancy and how does that affect the synthesis and secretion to thyroid hormones
High TBG
low T3 resin uptake
low levels of free T3 and T4 since all our bound
therefore increase in production and secretion of T3 and T4
even with high levels of T3 and T4, the person is said to be clinically euthyroid because they are all bound
What is the role of TSH and how is it regulated
used for the growth of the thyroid gland (tropic effect) and secretion of thyroid hormones
secreted by the anterior pituitary gland
regulated by TRH via the hypothalamus
and by the amount of free T3
this occurs at a steady rate
What are the stimulatory factors of thyroid hormone secretion
TSH
Thyroid stimulating immunoglobulins
increased TBG levels (pregnancy)
What are the inhibitory factors of the thyroid hormone secretion
I- deficiency
Deiodinase deficiency
Excessive I- intake (Wolff-Caikoff effect)
Perchlorate and Thiocynate (inhibit na and I cotransporter)
Propylthiouracil (PTU) inhibits peroxidase
Decreased TBG levels liver disease
What is the purpose of the thyroid hormone
helps synthesize a vast araray of proteins
including Metabolic enzymes in liver and adipose tissue
Na/KK ATPase synthesis
and B adrenergic receptors and myosin in the heart
Matures the CNS
Does growth promoting bone formation
WHat does thyroid hormones do to metabolic rate
Increase Basal metabolic rate by producing Na+/K+ ATPase
leads to o2 consumption and heat production
How does thyroid hormones affect lipid metabolism
stimulate fat mobilization and increases FA in the plaasma
enhances FA oxidtion
decreases blood cholesterol and triglycerides
helps convert carotene to vitamin A
How does thyroid hormones affect carbohydrate metabolism
increases Gluconeogenesis and glycogenolysis to generate glucose
enhances insulin dependent entry of glucose into the cell
How does T3 affect Cardiac aoutput
Increases cardiac output by incrreasing preload via renin-angiotensin-aldosterone
decreases afterload
increases cardiac chronotropy and inotropy
How does the thyroid hormone affect the cardiomyocyte
increases production of B1 receptors making the cell more sensitive to stimulation via the sympathetic nervous system
Symptoms of Hyperthyroidism
Increased BMR Weight loss heat intolerance osteoporosis agitation anxeity sweting tachycardia atrial fibrillation diarrhea high output heart failure
Primary Hyperthyroidism
Graves disease
decrease in TSH
but there is TSH immnoglobulins that are stimulating TSH receptors without the presence of TSH these are called (TSI)
Major clinical signs: Exophthalamos due to the anti TSH receptor antibodies
elevated levels of T4 and T3
presence of circulating TSI
has goiter
Secondary Hyperthyroidism
TSH secreting pituitary
similar issues as primary except Exophthalamos
Hypothyroidism symptoms
cold intolerance weight gain decrease in BMR stunted growth cretinism dry skin bradycardia decress cardiac output heart failure constipation
Causes and treatments of hypothyroidism
gland destruction: Hashimotos thyroiditis inhibition of thyroid hormone synthesis agenesis hypothalamic disease pituitary disease (Seehans syndrome)
Replacement doses of T4
- higher doses required for adults since metabolism of T4 decreases with age
- in women that have gone through menopause overprescribing T4 can lead to development of osteoporosis
Hashimotos THyroiditis
Thyroid hormone synthesis is impaired by thyroglobulin or TPO antibodies leading to decreation of T3 and T4 secretion
TSH levels are high
-Goiter (tropic effect)
Congenital Hypothyroidism
Iodide deficiency
impaired development of thyroid gland
deficit in thyroid hormones
Untreated leads to respiratory problems cretinism protruding tongue mental retardation jaundice Dry skin growth issues
Seehan Syndrome
Postpartum hypopituitarism due to necrosis of the pituitary gland
difficulties lactating
other endocrine issues may be present
Amenorrhea is present (absence of menstration)
Goiter
can develop from imbalances of the HPT axes
can be found in graves disease
Hyperthyroidism
and Primary hypothyroidism
-lack of Iodine in the diet
