Thyroid function tests
Standard Tests
TSH levels
Free T4 (fT4) levels
Measurements of total T4 + T3 used to be common however detects both bound and free T3 + T4
Elevated total T4 may occur in healthy individuals if there is an increase in binding protein concentrations
Reliable tests now exist for free T4 + T3
T3 = 3.9-6.7 pmol/L
T4 = 12-22 pmol/L
Thyroid-stimulating hormone
Produced by the pituitary gland, not the thyroid, however:
TSH levels are controlled by negative feedback – can be indication of thyroid function (changes in T3+T4 will result in changes in TSH to try compesate)
TSH levels greatly elevated in hypothyroidism – >10 fold increase over reference values
More sensitive marker than decreased fT4 - increased TSH occurs before fT4 decreases
TSH levels greatly supressed in hyperthyroidism
Low concentrations can also occur in non-thyroidal illness
TSH measurement is the first-line test of thyroid function.
Free T4 + T3 Measurements
Desirable as free hormone is clinically relevant
Total levels can change under conditions that alter thyroxine-binding globulin (TBG) levels e.g. pregnancy
Large changes in TBG may still affect fT4 + fT3 levels
fT3 levels often normal in hypothyroidism
fT3 levels usually raised more than fT4 levels in hyperthyroidism
Unless complicated by an illness effecting conversion of T4 to T3
Therefore: – fT4 levels are a better indication of hypothyroidism
fT3 levels are a better indication of hyperthyroidism
