2. Thyroid function tests
- Specimen collection :
• Specimens are routinely collected in a clot tube, although
anticoagulants can be used .
• Specimens free of lipemia or hemolysis are preferred .
3.
4. The tests used to investigate thyroid
function can be grouped into:
• Tests that establish whether there is thyroid
dysfunction ( TSH,T4 and T3 measurements)
• Tests to know the cause of thyroid dysfunction
(thyroid auto-antibody and serum hyroglobulin
measurements, thyroid enzyme activities, biopsy of
the thyroid, ultrasound and isotopic thyroid
scanning )
5. TSH :
- The single most sensitive, specific and reliable test of thyroid
status .
- In primary hypothyroidism [TSH] is increased.
- In primary hyperthyroidism, [TSH] is decrease .
6. Total T4 and Total T3 :
More than 99% of T4 and T3 circulate in
plasma bound to protein .
-Both [total T4] and [total T3] change if [TBG] alters
e.g. in pregnancy .
7. :Free T4 and Free T3
Free thyroid hormone concentrations are independent of
changes in the concentration of thyroid-hormone
binding proteins → more reliable
for diagnosis of
thyroid dysfunction.
8. Interpreting results of thyroid function tests:
:Primary hyperthyroidism
- Plasma [TSH] : ↓ due to feedback inhibition on the
pituitary
-- Plasma free and total T4 and T3 concentrations :
↑In a very small percentage of hyperthyroid patients,
plasma [total T4] and [freeT4] are both normal,
whereas both plasma [total T3] and [freeT3] are
increased ; this condition is known as
T3 hyperthyroidism or T3 thyrotoxicosis .
9. Primary hypothyroidism :
Plasma [TSH] : ↑
Plasma [free T4] and [total T4] : ↓
Plasma free T3 and total T3 measurements are
of no value here, since normal concentrations
are often observed .
10. Plasma [TSH] : abnormal
Thyroid hormone levels : normal
[TSH] : low → sub clinical hyperthyroidism
[TSH] : elevated → sub clinical hypothyroidism
Before the diagnosis of sub clinical thyroid disease can
be made, causes of an abnormal plasma [TSH] other
than thyroid disease must be excluded
eg : - pregnancy
- drug treatment
Sub clinical primary thyroid disease :
11. :Secondary thyroid disorders
Central (pituitary) hypothyroidism :
[TSH] & thyroid hormone levels → low
Hyperthyroidism due to a TSH-secreting tumor
very rarely
