Free thyroxine, or free T4, refers to a type of thyroid hormone in the body that isn’t connected or bound to certain proteins. Many people are also familiar with this term from laboratory testing for thyroid hormone dysfunction. The test to measure unbound levels of thyroxine is usually called a free thyroxine or free T4 test. In most cases, the measurement of this unbound hormone plus an analysis of thyroid stimulating hormone (TSH) can help accurately diagnose hyperthyroidism or hypothyroidism.
T4 present in the body is of two specific types. Part of the thyroxine produced becomes chained to proteins, and it no longer performs its necessary functions. Only the T4 that is free and unbound can do the work for which it was designed. This work includes metabolizing fat, protein, and carbohydrates, and regulating the body’s metabolic rate. Free thyroxine also helps the central nervous system by influencing the production of certain neurotransmitters, and this, in turn, affects other systems of the body.
It’s interesting to note that these effects are provided by about 1% of the thyroxine that is produced. Almost all of the T4 is bound. The free thyroxine represents a tiny amount of total T4. Nevertheless, analysis of this fractional proportion is usually the best measure of thyroid function.
A free thyroxine test is a simple blood test to measure unbound T4, usually along with a TSH measurement to help confirm any diagnosis. This blood test takes a minute or less for most people, and ordinarily doesn’t require fasting or any other restrictions. Lab results are analyzed and returned to doctors in a week or two.
Patients may also receive a record of their free thyroxine results. It will generally say whether TSH and T4 results were high, low, or normal, and will comment on how the test should interpreted. There may also be categories for borderline high or low readings that might be suggestive of thyroid problems.
Generally, a high TSH level combined with low T4 corresponds to hypothyroidism. Alternately, high free T4 and low TSH may indicate hyperthyroidism, or too much thyroid hormone. Borderline low and high results may or may not suggest a condition that needs treatment. If a patient appears symptomatic, some form of intervention might be indicated. Those individuals who have had borderline results in the past and develop symptoms should also ask doctors for another free thyroxine test.
How or when to treat minimally high or low levels of thyroid dysfunction is a question that is currently not resolved. For example, people with bipolar disorder often have lower free thyroxine levels and some argue that this should always be treated, even if it is borderline. Since unbound T4 has a regulatory effect on neurotransmitters, there is some logic to this line of reasoning. Presently, the medical community is not united on how to interpret borderline thyroid testing, and there is strong disagreement about whether or when such results should indicate medical treatment.