Thyroid Hormones - Blog Posts

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

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