Thyroid Function Tests Flashcards
What does TSH and TRH stand for?
Where is TSH released from?
Where is TRH released from?
Thyrotropin-releasing hormone (TRH), is a hypophysiotropic hormone, produced by neurons in the hypothalamus, that stimulates the release of thyroid-stimulating hormone (TSH) and prolactin from the anterior pituitary.
Look at page 216 in clinical med book
Which thyroid hormone is produced more? - T3/4 (thyroxine)
They are both bound to TBG. What does it stand for?
T4 (thyroxine)
it is 5-fold less active than T3 but it made mostly by peripheral conversion of T4
Thyroxine binding globulin - they active when unbound
Basic tests:
Why is free T3/4 more useful than total T3/4?
Increases in TBG:
- Why does pregnancy increase it?
- Why does oestrogen therapy increase it?
Decreases in TBG:
- Why does nephrotic syndrome decrease it?
- Why does malnutrition decrease it?
- Why do corticosteroids decrease it?
- Why does chronic liver disease decrease it?
- Acromegaly can also cause it!!
Same as above
Total T3/4 is affected by TBG.
Oestrogens stimulate expression of TBG in the liver, and the normal rise in oestrogen during pregnancy induces roughly a doubling in serum TBG concentrations.
It causes loss of TBG in urine
Less protein being absorbed
Steroids reduce protein synthesis in the liver
Less protein synthesis in the liver due to damage
Basic tests:
What will follow the total T3 and T4 in a test? - meaning if it is low, ___ will also be low and vice versa
If hyperthyroidism is suspected T3, T4 and TSH is tested. Why is TSH low?
If hypothyroidism is suspected or monitoring replacement Rx:
- Why is T3 not tested?
- For monitoring, why is the test done at the same time?
This not the case if they have a TSH-secreting pituitary adenoma.
TBG
Because the pituitary gland will try to compensate for the excess T3 and T4 will stop producing TSH in an attempt to stop production of the thyroid hormones.
The thyroid only makes small amounts of T3. Even in cases of severe hypothyroidism, T3 levels don’t go down that much. T4 is produced in large quantities by the thyroid. However, TSH is a far superior screening test because small changes in T4 cause large TSH spikes.
TSH varies throughout the day:
- Lowest at 2pm
- 30% higher in darkness
Basic tests:
What is sick euthyroidism?
During illness, TFT’s tend to be deranged.
The typical pattern is for everything to be low.
Hormone profile and diagnosis:
Hypothyroidism - what hormones are tested and what are the results?
Why may T4 be normal but TSH be high in subclinical hypothyroidism?
What results would you expect for a TSH secreting tumour?
Hyperthyroidism - what hormones are tested and what are the results?
What results would you expect in subclinical hyperthyroidism”
TSH - low
T4 - high
When the thyroid gland becomes inefficient such as in early hypothyroidism, the TSH becomes elevated even though the T4 and T3 may still be within the “normal” range.
TSH-secreting tumour or thyroid hormone resistance
TSH - low
T3 - high
T4 - high
Low TSH but normal T3 and T4
Hormone profile and diagnosis:
What would you expect with hypothalamic and pituitary disorders?
What antiarrhythmic drug could cause thyroid disease and why?
Low TSH
Low T4
Amiodarone
Due to it’s high iodine content and its direct toxic effect on the thyroid.
Other tests:
Thyroid autoantibodies:
- What type of diseases are these used in?
- TPO is usually tested. What does this stand for?
TSH receptor antibody:
- What hyperthyroidism cause is this used to test for?
Serum thyroglobulin is used to monitor the Rx of _____.
What is the blank?
Antithyroid Peroxidase antibodies
Autoimmune diseases:
- e.g. Graves/Hashimoto’s
Graves
Thyroid cancers
Thyroglobulin is the other major component needed for synthesis of thyroxine and triiodothyronine. Thyroglobulin is the matrix for thyroid hormone synthesis and is the form in which hormone is stored in the gland.
Other tests:
What scan can be done for cystic nodules, malignant nodules and goitres?
Isotope scan:
- Why type of radioisotope is used?
- How does it work?
- What is the use?
Radioactive iodine
This is because your thyroid gland uses iodine to make certain hormones in the body. So, when the radioactive iodine is given, it is quickly taken up by the tissues of your thyroid gland. Cells which are most active in the target tissue or organ will take up more of the radionuclide. So, active parts of the tissue will emit more gamma rays than less active or inactive parts.
Looks at the cause of hyperthyroidism
Detects retrosternal goitre (behind the sternum)
Looks a thyroid metastases
