Thyroid Function Testing Clinical Chemistry Review 111
PRIMARY SECONDARY
TEST HYPOTHYROIDISM HYPOTHYROIDISM HYPERTHYROIDISM T 3 THYROTOXICOSIS COMMENTS
Thyroid-stimulating
hormone (TSH)Free T 4
(FT 4 )Free T 3
(FT 3 )↑
↓
↓
↓
↓
↓
↓
↑
↑
↓
N
↑
1st test for screen-
ing. If normal, no
further testing.Biologically active
form of T4. 2nd
step in screening if
TSH abnormal.Biologically active
form of T3. Usually
not helpful in Dx
of hypothyroidism
because last test
to become abnor-
mal. Usually only
tested when TSH is
↓& FT 4 isn’t ↑.Primary hypothyroidism = thyroid insufficiency. Secondary hypothyroidism = pituitary insufficiency. Graves’ disease (type of autoimmune disease) is most common cause of
hyperthyroidism. Thyroid tests are immunoassays.