Study your flashcards anywhere!

Download the official Cram app for free >

  • Shuffle
    Toggle On
    Toggle Off
  • Alphabetize
    Toggle On
    Toggle Off
  • Front First
    Toggle On
    Toggle Off
  • Both Sides
    Toggle On
    Toggle Off
  • Read
    Toggle On
    Toggle Off

How to study your flashcards.

Right/Left arrow keys: Navigate between flashcards.right arrow keyleft arrow key

Up/Down arrow keys: Flip the card between the front and back.down keyup key

H key: Show hint (3rd side).h key

A key: Read text to speech.a key


Play button


Play button




Click to flip

105 Cards in this Set

  • Front
  • Back
What are the hormones secreted by the thyroid?
-Thyroxin T4
-Triiodothyronine T3
What cells secrete T3/T4? How much of each?
Follicle cells
93% T4
7% T3
What cells secrete calcitonin?
Parafollicular cells
Which thyroid hormone is more physiologically active?
What is colloid made of?
What ingredient is absolutely essential for Thyroxine formation?
Iodine in the form of iodide
Where do we get our iodized iodine?
Table salt
Where is iodine absorbed?
In the GI tract
What is the first step in forming thyroid hormones?
Iodide trapping
What is responsible for Iodide trapping?
A specific active transporter on the basal membrane of thyroid follicle epithelial cells.
What does the pump do to intracellular iodide conc
-At normal activity
-At maximal activity
Normal: increases the [I] to 30x the blood concentration
Max Activity: to 250X it!
What is responsible for stimulating the iodide pump?
What do the ER and golgi apparatus of thyroid epithelial cells secrete?
The glycoprotein Thyroglobulin
What does thyroglobulin consist of?
~70 tyrosines
Where do the thyroid hormones form and get stored?
WITHIN the thyroglobulin molecules
What must happen to iodide in order to form thyroid hormone?
It must be oxidized to Iodine so that it can combine with tyrosine
What is responsible for this important oxidation of iodine?
Peroxidase and hydrogen peroxide.
Where is peroxidase located?
Within the apical membrane or attached to it.
So where is iodine oxidized?
At the point where thyroglobulin enters the colloid.
What will happen if peroxidase is absent or blocked?
No thyroid hormone gets made.
What is the binding of iodine to thyroglobulin called?
Organification of thyroglobulin
What enzyme makes organification in thyroid cells fast?
When thyroglobulin is released from the Golgi and into the colloid, how much gets iodinated immediately? What are the products called?
1/6 of the Tyrosine residues get organified immediately --> forms MIT + DIT
So what makes T4?
What makes T3?
How many T4 can one thyroglobulin molecule store?
Up to 30
How much T4 does the thyroid store at one time?
Enough to maintain sufficient blood levels for 2-3 months in the absence of hormone production.
What is the effect of the vast amount of thyroid hormone stored?
Physiologic effects of deficiency won't be seen for a few months.
What must occur for thyroglobulin to release T3/T4?
-Pinocytosis from colloid
-Lysosome fusion w/ vesicle
-Protease cleaveage of the hormones from thyroglobulin
How do free T3/T4 get from the epithelial cell to the blood?
By diffusion
Does all iodinated Tyrosin on thyroglobulin become thyroid hormone?
No; actually 75% doesn't
What happens to the iodinated tyrosines on thyroglobulin that don't make it to the hormone level?
They get deiodinated after digestion in lysosomes, and iodine is once again put into the colloid.
What results from congenital absence of Deiodinase enzyme?
Iodine deficiency due to lack of recycling.
How much iodized iodine do we normally take in form the diet?
500 ug
How much of that iodine goes to the thyroid?
~120 ug
Where does the other 380 ug go?
To the urine
Of the 120 ug iodide that goes to the thyroid how much gets put into thyroid hormones?
70 ug
Where does the other 50 ug form the thyroid go?
To the urine
What happens to the iodine after thyroid hormone exerts its effects on target tissues?
It gets metabolized and excreted
How much metabolized iodine from thyroid hormones gets excreted in the urine? feces?
Urine = 55 ug
Feces = 15 ug
So of the 500 ug iodide that we taken in daily how much do we excrete?
What is the half life of
T4 = 6.2 days
T3 = 1 day
Why do the thyroid hormones have such a long half life?
Because they circulate bound to plasma proteins and are protected from degradation.
What are the 3 plasma proteins that bind Thyroid hormones?
-Thyroxine binding globulin TBG
-TBPA (transthyretin)
Of the total free T4 pool how much is bound to protein? What about for T3?
99.97% - T4
99.7 - T3
So does that mean that there is more free T3 than T4?
No; there is much less T3 than T4 circulating, just a higher percentage is free.
Why is there more T3 free than T4?
Because it is bound less tightly by proteins
Does TBPA bind T3?
No - it is thyroxine binding pre-albumin
How does the fact that thyroid hormones are protein-bound affect their action?
They have a SLOW ONSET and LONG DURATION of action
How long does it take to see a metabolic rate increase after injecting a large amt of T4?
About 2-3 days
How many days does it take for the injection to reach its max effect?
10-12 days
How many days does it take for metabolic rate to return to normal?
If you were to give the same large dose but of T3 instead of T4, how would the response time differ?
Metabolism rate would increase in 6-12 hours and max at 2-3 days!! much faster
In a nutshell what is the general effect of Thyroid hormone?
Increased transcription of large numbers of genes.
In what body cells does this increase in transcription occur?
Virtually all of them
What happens to thyroxine before it exerts its effect on gene transcription?
One iodine is removed to form T3
Why is T4 converted to T3 before exerting its effect?
Because intracellular thyroid hormone receptors have a much higher affinity for T3
Where is most thyroid hormone receptor found?
On DNA Thyroid Hormone Response Elements where it is complexed with Retinoid X Receptor (RXR)
What happens when T3 binds its receptor on the thyroid hormone response element?
It increases transcription, translation, and new protein synthesis.
What is the intracellular enzyme that converts T4 to T3 before binding its receptor?
What broad effect does thyroid hormone gene activation have on the body?
Increased metabolic rate
What are 4 specific systems that are affected by TH?
-CNS development
-CV system
If target cells can make T3 from T4, can they make T4 from T3?
Nope; only the one way of T4 to T3
How does thyroid hormone affect metabolism?
It increases food breakdown - both carb and proteins.
What is Cretinism?
Neonatal mental retardation due to a lack of Thyroid hormone
What must be done to prevent Cretinism?
Thyroid hormone administration of levothyroxine given immediately at birth.
What results if thyroid hormone is not given at birth?
Irreversible failure of CNS development.
What is an adverse effect of overproduction of thyroid hormone and overstimulation of metabolic rate?
Relative vitamin deficiency
How is the target of thyroid hormone within the nervous system?
The sympathetic adrenergic system
How does thyroid hormone affect the symp adrenergic system?
It increases B-adrenergic receptors in the heart, liver, muscle, and fat cells.
What is the effect of increased b-adrenergic receptors?
Increased b-adrenergic responses
What is the main regulatory hormone that affects thyroid secretion?
TSH from the pituitary gland
What type of hormone is TSH?
A glycoprotein
What exactly does TSH do?
Increases all known secretory activities of the thyroid glandular cells
What is the most important early affect of TSH?
Initiation of Thyroglobulin proteolysis and release of T3/T4
Where are the TSH receptors on the thyroid gland?
On the basal surface of follicular cells
What is the direct effect of TSH binding its receptor?
Increased cGMP
So the 2 resulting effects of TSH stimulation of the thyroid:
1. Immediate release of TH
2. Prolonged growth of the thyroid glandular tissue itself
(Tropic + Trophic)
What is the major controller of thyroid hormone release?
What is the major controller of TSH release?
T3/T4 - via negative feedback
What is the role of TRH then?
To adjust the setpoint of the pituitary. When more T3/T4 is needed than normal, TRH will increase, and vice versa.
What is primary hypothyroidism?
Decreased thyroid hormone due to a problem with the thyroid gland itself.
What is the name for primary hypothyroidism?
Hashimoto's thyroiditis
What happens in Hashimotos?
An autoantibody develops against the thyroid and causes destruction of the gland.
What is the basis for diagnosing Hashimoto's?
Low free T4 + High TSH
What is 2ndary hypothyroidism?
Decreased thyroid hormone due to decreased TSH - hypopituitarism
What is the basis for diagnosing 2ndary hypothyroidism?
Low Free T4 + normal/low TSH
What is the treatment for low thyroid hormone (whether primary or secondary)?
What is the most common form of hyperthyroidism?
Primary - an overproduction due to a problem with the gland itself.
What is primary hyperthyroidism called?
Grave's disease
What is the pathology of Grave's?
An autoantibody to the TSH receptor develops and results in overstimulation of thyroid hormone release.
What is the basis for diagnosing Grave's disease?
-High Free T4
-Low TSH
What is an interesting sign of Grave's disease?
The pemberton sign - guy turn's red when he raises his arms.
So the best screening test for diagnosing a thyroid disorder is:
Serum TSH
Graves disease:
-Total T4
-Free T4
Total T4 = High
TBG = normal
Free T4 = high
TSH = low
Hashimoto's Thyroiditis:
-Total T4
-Free T4
Total T4 = Low
TBG = normal
Free T4 = Low
TSH = High
-Total T4
-Free T4
Total T4 = High
TBG = High
Free T4 = N
Why do TBG/T4 increase in pregnancy?
Because estrogen stimulates the liver to make more TBG, which stimulates an increase in T4.
What are 3 types of treatment for hyperthyroidism?
1. Radioactive Iodine injection
2. Surgical removal
3. Drugs
What does radioactive iodine injection do?
Destroys the thyroid gland - it melts!
When is radioactive iodine treatment bad? Why?
In pregnant women - it crosses the placenta
So what should you ALWAYS do before prescribing radioactive iodine treatment?
A pregnancy test (in women of course)
What is the only certified drug for treating hyperthyroidism in pregnancy/
Why is drug treatment not the preferable method?
Because as it blocks organification of thyroglobulin, the resulting decrease in T3/T4 results in increased TSH release - always chasing your tail.
What type of treatment is most commonly used for thyroid cancer?
Surgical thyroidectomy