16. Thyroid Gland Flashcards
What is the anatomy of the thyroid follicle (the functional unit of the thyroid)?
Surrounded by a layer of epithelial cells and the lumen is filled w/ colloid.
What are C cells? What do they secrete?
Parafollicular cells.
Secrete calcitonin.
Can the epithelial cell size and amount of colloid change?
Yes, with activity.
What synthesizes thyroid hormone?
Follicular epithelial cells
What is colloid composed of?
Newly synthesized thyroid H attached to thyroglobulin.
What is produced in more quantity: T4 or T3?
10x more T4 is produced than T3
What element is rich in thyroid H?
Iodine
What catalyze the conversion of T4 to T3? Where does it happen?
Deiodinase.
80-90% in the periphery.
About 10-20% is secreted from the thyroid.
What can reduce the conversion of T4 to T3? (3)
Fasting
Medical/surgical stress
Catabolic diseases.
Deficiency of deiodinase mimics a dietary deficiency of what?
Iodine deficiency
What is favored favored when iodine levels are low?
T3 formation is favored when Iodide is low.
What is an effective treatment for hyperthyroidism?
PTU
What is the Wolff-Chaikoff effect?
When high levels of I- inhibit organification and synthesis of thyroid H.
How much of iodide is “trapped” in the thyroid?
How much is leaked from thyroid to the ECF?
120 ug is trapped.
60 ug is leaked.
How is iodine stored?
Iodinated as tyrosines of thyroglobulin.
How much thyroid H is secreted daily from a follicular cell?
60 ug
How can radioactive iodine be helpful?
It can help assess the activity of the thyroid.
-ex: hyperthyroidism, high uptake of radioactive iodine b/c high thyroid hormone production
The majority (99%) of thyroid H circulates in the bloodstream how?
Bound to plasma proteins (TBG, albumin, transthyretin)
1% is free.
What are the 3 main thyroid hormone binding proteins? (3)
Thyroid binding protein (TBG) - 70%
Transthyretin (TTR) - 10-15%
Albumin - 15-20%
Half life of T4 and T3
T4 - 6 days
T3 - 1 day
Most circulating thyroid H is:
T4
What does the T3 resin uptake test examine?
Tests circulating levels of TBG.
Hyperthyroidism and T4/T3 resin uptake levels
Increase in T4
Increase in T3 resin uptake
Hypothyroidism and T4/T3 resin uptake levels
Decreased T4
Decreased T3 resin uptake
Hepatic failure and TBG and T3 resin uptake levels
Decreased TBG
Increased T3 resin uptake
Pregnancy and TGB and T3 resin uptake levels
Increase in TBG
Decrease in T3 resin uptake
Hepatic failure causes a ________ in levels of TBG. How does that effect FREE T3, T4 levels?
Decrease.
Increases free T3/4, followed by inhibition of synthesis (neg feedback).
Pregnancy causes an ________ in TBG levels.
How does that effect levels of bound T3/4, free T3/4 and synthesis of T3/4?
Increase in TBG.
Increases bound T3/4.
Decreases free T3/4.
Increases synthesis of T3/4.
What does “clinically euthyroid” mean?
In pregnancy when there is a increased levels of T3/4, but active (free levels) of TH is normal.
What 2 things does TSH regulate?
Growth of the thyroid.
Secretion of THs.
What regulates TSH? (2)
TRH and free T3
At what rate does TSH secretion occur?
Steadily
What organ does T3/4 inhibit?
Ant pit and hypothalamus if levels are high
What’s the second messenger for TSH at the thyroid?
Cyclic AMP
Thyroid hormones can cause the synthesis of new:
Proteins. (Nak atpase, receptors, enzymes, myosin, Ca atpase)
What is the effect of TH on BMR? How does it do that?
Increases basal metabolic rate.
Increases activity of NaK ATPase —> increased O2 consumption.
What is the correlation of cholesterol/TAG plasma concentration w/ TH?
Inversely.
High cholesterol —> low thyroid.
How do hypothyroid patients suffer from blindness/yellow skin?
Thyroid h is necessary for conversion of carotene to vit A
What is the effect of TH on the mobilization of fats?
Increases fat mobilization.
Whats the effect of TH on carb metabolism?
Increases GNG and glycogenolysis to free glucose..
Enhances insulin dependent entry of glucose into cells.
How does T3 increase cardiac output?
Activates RAAS —> Increase preload —> increases CO
What receptors do thyroid hormones stimulate the synthesis of?
Cardiac B1 adrenergic —> chronograph and inotropy.
Myocardium is more sensitive to SNS when TH is high.
Low TH in perinatal development can lead to:
Abn synapsing.
Decreased dendritic branching/myelination.
Primary hyperthyroidism is known as:
Graves’ disease
What is an example of secondary hyperthyroidism?
TSH-secreting pituitary
What happens to TSH levels in hyperthyroidism?
Decrease due to negative feedback of T3 on the ant pit.
IF the defect is in the ant pit, then TSH will in crease.
What do thyroid-stimulating immunoglobulins do?
Stimulate the TSH receptor without TSH around. (Graves’ disease).
How does thyroid-stimulating immunoglobulin effect TSH levels?
TSH levels decrease because high TH inhibits TSH secretion.
Clinical signs of Graves’ disease
Exophthalamos (protrusion of eyes), periorbital edema.
How is Graves’ disease diagnosed?
Elevated serum free and total T3/4 and clinical signs of goiter and ophthalmophaty.
Primary causes of hypothyroidism
Agenesis
Gland destruction
Inhibiting TH synthesis/release.
Pituitary disease.
How is hypothyroidism treated? How does is change with pt age?
Replacement doses of T4.
Younger patients need higher doses because they metabolize it quicker.
Overprescribing T4 in postmenopausal women can cause:
Osteoporosis
Hashimoto’s thyroiditis
TH synthesis is inhibited by thyroglobulin or TPO ABs —> low T3/4.
TSH is high.
Cretinism is caused by:
Untreated postnatal hypothyroidism.
Hypothyroidism and iodine deficiency
Iodine deficiency leads to decrease in synthesis of TH.
TSH levels increase.
Can have a goiter.
Goiter develops in response to both:
Hyperthyroidism and primary hypothyroidism.
