What is the only organ that synthesizes T4?
Thyroid
Where does 80% of circulating T3 derive from?
T4
Only _____ T3 and T4 can enter cells
free (unbound)
What tests make up the thyroid panel?
- TSH
- Total T4
- Total T3
- Free T4
- Free T3
- Thyroxine binding globulin
What stimulates the thyroid to produce T4&T3?
TSH (Thyroid Stimulating Hormone)
What is the first test you should order for suspected thyroid disorder?
TSH
Where is TSH secreted from?
Anterior pituitary
What factors can cause an increased TSH?
- Primary HYPOTHYROIDISM
- Thyroiditis
- Large doses of iodine
- Severe and chronic illnesses
- Pituitary TSH-secreting tumor
What factors can cause a decrease in TSH?
- Secondary hypothyroidism (pituitary dysfunction)
- HYPERTHYROIDISM
- Pituitary hypofunction
What is total T4 a measure of?
- Amount of T4 that is bound to protein and free
- Direct measurement of T4 in the blood serum
- Used to rule out hyper and hypothyroidism
- Range = 4-12
What can cause an increased level of T4?
- Primary HYPERTHYROIDISM (Grave’s disease, toxic thyroid adenoma)
- Acute thyroiditis
- Pregnancy
- Thyroid cancer
- Toxic multinodular goiter
What can cause a decrease in total T4 levels?
- HYPOTHYROID state (cretinism, surgical ablation of the thyroid, Myxedema)
- Pituitary insufficiency
- Hypothalamic failure
- Protein depleted states (nephrotic syndrome)
- Renal failure, cirrhosis, cushing disease, surgery, advanced cancer)
T3 is more _____ than T4, but its effects are _____
Active, shorter
What percent of T3 is bound to proteins?
70%
What percent of thyroid hormone is T3?
7-10%
What some causes of an increased T3?
- Primary Hyperthyroidism (Grave’s disease, toxic thyroid ademona)
- Acute thyroiditis
- Pregnancy
What are some causes of a decreased T3?
- Hypothyroidism
- Hypothalamic failure
- Protein malnutrition and other protein depleted states
- Iodine insufficiency
- Liver disease (this is where T4 gets converted to T3)
What is free T4?
Amount of thyroxine that is unbound in the blood
- Active and available to tissues (gets converted to T3)
- *Controls metabolism**
What is free T3?
Amount of triiodothyronine that is unbound in the blood
- Active
- Controls metabolism
What is thyroxin-Binding globulin?
Carries T3 and T4 in the blood stream (this is the protein that T3 and T4 bind to)
What directly impacts TOTAL T3 and T4 concentration but not FREE T3 and T4?
-Amount of TBG impacts total T3/T4 but not free T3/T4 because it is not bound to it
When do we order thyroid testing?
- Suspect patient to have hypothyroidism
- Suspect patient to have hyperthyroidism
- Abnormal thyroid exam
Who do we screen?
If people are complaining of symptoms -> TSH
- Normal TSH = no further testing
- High TSH = free T4 to determine degree of hypothyroidism
- Low TSH = free T4 and T3 to determine degree of hyperthyroidism
When would you order both TSH and T4 right away?
- If pituitary or hypothalamic disease is suspected (ie young woman who hasn’t gotten her first period yet)
- If TSH comes back normal but there is high suspicion of hyper or hypothyroidism
