New research supports the current two-step approach to assessing thyroid function – thyroid-stimulating hormone (TSH) levels first then free thyroxine (fT4) only if TSH is outside the reference range.

An analysis of more than 4,000 participants in the Busselton Health Study found the two-step approach could eliminate unnecessary fT4 testing in up to 93% of participants compared to simultaneous testing.

The study said clinicians were concerned that the two-step approach may miss some patients with thyroid dysfunction despite a normal TSH.

However 96% of those with normal TSH also had normal fT4 levels – a finding consistent with other studies.

Importantly in patients with fT4 levels outside the reference range, 85% were within 2 pmol/l of the limits of the fT4 reference range.

“We consider these participants likely to be euthyroid, healthy outliers,” the study said.

“Taken together, ours and previous data show that a sole, normal TSH test reliably diagnoses euthyroidism and does not miss patients with clinically relevant thyroid dysfunction.”

The study said there was however considerable variation in practice.

The researchers attempted to develop a prediction score that might help detect patients at risk of thyroid function and effectively provide ‘a novel three -step approach’.

However the best model of predicting hypothyroidism only explained about 5% of the variation and a clinical risk score to detect hyperthyroidism was found not to be feasible.