Diagnostic Use
If Hyperthyroidism is suspected:
Measure Thyroid Stimulating Hormone (TSH) and Free Thyroxine (FT4): If the TSH is suppressed and the FT4 is increased, no further tests are required.
If the TSH is suppressed and FT4 is normal, measure Free Triiodothyronine (FT3). An increased level of FT3 confirms the diagnosis.
Monitoring Hyperthyroidism Treatment:
FT4 (or FT3 if abnormal) may be useful in the early stages of treatment. TSH should return to the reference range, but this may take 2 – 3 months to achieve.
If Hypothyroidism is suspected:
Measure TSH – If TSH is high, then no further tests are required to make the diagnosis of primary hypothyroidism.
If TSH is low or “normal”, then secondary hypothyroidism may be the cause.
In secondary hypothyroidism of pituitary origin, the TSH is typically “normal” (not low) and the free T4 is low.
Monitoring Patient on Thyroxine Therapy:
Maintain TSH within the reference range (primary hypothyroidism) and free T4 in the high normal range (secondary hypothyroidism).
Acutely Unwell Patients:
Temporary hypothalamic/pituitary dysfunction (aka ‘sick euthyroid syndrome’) may occur with acute illness and can create a confusing thyroid function pattern. Therefore, it is generally NOT recommended to assess thyroid function when a patient is acutely unwell, unless it is believed that thyroid dysfunction is the basis of the illness.
See separate entries for TSH, T3 and T4 for more information