Thyroxine (T4)

THYROXINE, FREE (T4)

Testing free thyroxine (T4) levels is useful when assessing thyroid function and diagnosing thyroid diseases such as hyperthyroidism and hypothyroidism. High and low free thyroxine levels are linked to temporary and chronic thyroid diseases. Thyroxine is the storage thyroid hormone secreted from the thyroid into the circulation. It has to be converted into triiodothyronine (T3), which is active on a cellular level. Hence, measuring the levels of free T3 is also important when comprehensively assessing thyroid function throughout the system.

• Reference range: 9–19 pmol/L

No optimal range has been established in terms of mortality, the prevention of cardiovascular diseases, etc. Recommendations often suggest levels that fall in the reference range's middle or top third (not in the bottom third). The free thyroxine value in the healthy population is on average 15.07 ± 2.528 pmol/L when the reference range is 10.2–31 pmol/L. Thus, this value cannot be directly applied to the above reference range. However, it does suggest that the values of an individual not using thyroid medication should be near the middle of the reference range or closer to the upper limit.