Thyroid Function Tests Flashcards
What are the 4 functions of the thyroid gland?
- control metabolism
- regulate body temperature
- influence HR
- regulate protein production
The thyroid produces what 3 hormones?
- thyroglobulin (Tg)
- T4
- T3
Which of the 3 hormones produced by the thyroid is the most active form?
T3
Which of the 3 hormones is the most predominant hormone secreted by the thyroid?
T4
What substance is essential for the production of T3/T4?
iodine
What % of T3 is converted from the T4?
80%
*20% converted by thyroid
Describe the hypothalamus-pituitary-thyroid axis (cycle)?
Hypothalamus (TRH) > pituitary gland (TSH) > thyroid gland to produce T4+T3
production of T4+T3 down-regulates the hypothalamus + pituitary gland
What is the difference between, primary, secondary or tertiary thyroid disease?
- primary = problem at thyroid
- secondary = pituitary gland
- tertiary = hypothalamus
What is the normal range of thyroid-stimulating hormone (TSH)?
How long does it take TSH levels to adjust?
- 45-4.5 mIU/L
- take 6-8 weeks to adjust
What 3 medications can result in a false decrease of thyroid-stimulating hormone (TSH)?
- Dopamine
- High dose glucocorticoids
- Levodopa
What 2 medications can result in a false increase of thyroid-stimulating hormone (TSH)?
- Lithium
- Amiodarone
What is the best test to further evaluate an abnormal TSH?
What is the normal range
Free T4 (FT4)
Normal range = 12-30 pmol/L
T4 decreases with the use of what medication?
lithium
Free T3 (FT3) is valuable in evaluating patients with what condition?
Free T3 test is not recommended for the diagnosis of what thyroid condition?
- thyrotoxicosis
- hypothyroidism
What is the normal range of Free T3(FT3)?
- 0-7.0 pmol/L
* may decrease by <25% in older persons
If you have a high suspicion for hypothyroidism what test should you order?
TSH
If you have a high suspicion for hyperthyroidism what tests should you order?
TSH, FT4, FT3
Which of the thyroid function tests alone is good enough to monitor levothyroxine therapy?
TSH
TRH stimulation will increase TSH and thyroid function in what type of hypothyroidism?
(central) hypothalamic hypothyroidism
What lab findings help in the diagnoses of autoimmune hypothyroidism?
-Check for high levels of both:
anti-TPO & Tg antibodies
What % of subclinical hypothyroidism will progress to overt hypothyroidism?
55%
What substance is produced and used by the thyroid to make T3/T4?
Thyroglobulin (Tg)
What is the best initial screening test for thyroid functioning disorders?
TSH
