Thyroid stimulating hormone
TSH secretion is regulated by negative feedback from serum free
thyroid hormone levels. The serum TSH response to change in free T4
is logarithmic; a twofold change in free T4 induces inverse 10-100
fold changes in TSH. Thus, this negative amplified feedback as the
serum T4 increases or decreases, makes TSH a sensitive marker of
thyroid function which can become abnormal before there is a
diagnostic change in free T4.
Part of Profile
Thyroid Function Tests
Request on ICE
There is little value in doing thyroid function tests in acutely
ill patients as non-thyroidal illness may result in misleading
thyroid function test results. The test is therefore of
limited value in in-patients
It is normally not necessary to repeat TFT's within 4 weeks
Same Day (Monday to Friday)
Serum in yellow top (SST) tube
No specific collection conditions
Causes for Rejection
Unlabelled or inadequately labelled sample
0.27 to 4.2 mU/L.
Neonates may have levels up to 20 mU/L which decrease to normal
by 6 weeks of age.