Free thyroxine is an unbound thyroid hormone that helps regulate the body’s metabolic rate and affects the central nervous system. A blood test measuring free T4 and TSH can diagnose hyperthyroidism or hypothyroidism. Borderline results may or may not require treatment, and there is disagreement in the medical community about how to interpret them.
Free thyroxine, or free T4, refers to a type of thyroid hormone in the body that is not connected or bound to certain proteins. Many people are also familiar with this term from lab tests for thyroid hormone dysfunction. The test to measure levels of unbound thyroxine is usually called a free thyroxine or free T4 test. In most cases, measurement of this unbound hormone plus a thyroid stimulating hormone (TSH) test can help accurately diagnose hyperthyroidism or hypothyroidism.
T4 present in the body is of two specific types. Part of the produced thyroxine is chained to proteins and no longer performs its necessary functions. Only the free and unconstrained T4 can do the job for which it was designed. This work includes the metabolism of fats, proteins and carbohydrates and the regulation of the body’s metabolic rate. Free thyroxine also helps the central nervous system by affecting the production of certain neurotransmitters and this, in turn, affects other systems in the body.
Interestingly, these effects are provided by approximately 1% of the thyroxine produced. Almost all of T4 is bonded. Free thyroxine represents a small amount of total T4. However, 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 does not normally require fasting or other restrictions. Lab results are analyzed and returned to doctors in a week or two.
Patients may also receive a record of free thyroxine results. It will generally say whether your TSH and T4 results were high, low or normal and comment on how the test should be interpreted. There may also be categories for high or low limit values that could be indicative of thyroid problems.
Generally, a high TSH level combined with a low T4 corresponds to hypothyroidism. Alternatively, a 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 may 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 currently an unresolved question. For example, people with bipolar disorder often have lower free thyroxine levels, and some argue that this should always be treated, even if it’s borderline. Since unbound T4 has a regulatory effect on neurotransmitters, there is logic to this line of reasoning. Currently, the medical community is not united on how to interpret borderline thyroid tests, and there is strong disagreement about whether or when such results should indicate medical treatment.
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