TSH or Free T4 for Assessing Thyroid Function?

Peter Forster Best Practices, Physical Conditions and Health, Testing Leave a Comment

There are many reasons why thyroid status is important in a clinic treating people with mood disorders. For one thing, hypothyroidism and hyperthyroidism can mimic some of the symptoms of mood disorders. For another, lithium, a medication we often prescribe, is associated with hypothyroidism in a significant minority of patients taking the medicine.

One of the questions we wrestle with is which labs how to interpret the results of thyroid tests.

A 2020 review article in the prestigious journal Thyroid suggests that we may want to pay more attention to free T4 levels in assessing a patient’s clinical thyroid status.

TSH is the controlling hormone for the release of T4 and T3 from the thyroid gland. TSH levels increase as T4 and T3 levels decrease. This is because the brain attempts to activate the thyroid when blood levels of thyroid hormones fall.

In addition, TSH levels change earlier and more dramatically than T4 and T3 levels. In other words a TSH level below normal will be in earlier finding of hyperthyroidism than a T4 and/or T3 level above normal. Conversely a TSH level above normal will also be in earlier finding of hypothyroidism than a change in T4 and T3 levels.

This has led many physicians to focus on the measurement of TSH as a way of identifying people with hypo or hyperthyroidism.

This article suggests we reconsider that. While it is true that abnormal TSH levels precede changes in T4 and T3. The meaning of an isolated abnormal TSH level is unclear. Is there a reason why you might want to treat someone who only has an elevated TSH and the normal we T4 and free T3 with thyroid hormone?

Across many clinical studies it seems clear that the physiologic effects of low or high thyroid function correlate much more strongly to free T4 and free T3 levels than to TSH levels. In fact, correcting for changes in T4 and T3 levels there appeared to be no correlation between TSH level and body function.

Does this mean there is no role for checking a TSH level?

In our clinic we often check all three hormone levels. TSH is still useful as an early warning system for changes in thyroid function. If the TSH level is abnormal it suggests closer monitoring of thyroid function, but an isolated TSH abnormality with no clinical symptoms probably does not merit any intervention other than close monitoring.

For More Information…

Fitzgerald SP, Bean NG, Falhammar H, Tuke J. CLINICAL PARAMETERS ARE MORE LIKELY TO BE ASSOCIATED WITH THYROID HORMONE LEVELS THAN WITH TSH LEVELS: A SYSTEMATIC REVIEW AND META-ANALYSIS. Thyroid : official journal of the American Thyroid Association. 2020.

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