Questionable High Free T4 Concentrations: When Confirming against an Alternative Method Is Not Enough

Abstract

A 60-year old male was referred to the Endocrinology Department due to confusing results of thyroid tests during a routine medical check-up. The asymptomatic patient presented with increased free thyroxine (FT4) concentrations, and thyroid-stimulating hormone (TSH) and total thyroxine (T4) concentrations within the reference range (Table 1), with no tachycardia, distal tremor, visual impairment, or any other symptoms of thyrotoxicosis. He presented with a 10-kg weight gain over the previous few months (body mass index 27.7 kg/m 2 ), which was attributed to changes in dietary and exercise habits due to the mobility restraints related to control of COVID-19 dissemination. Physical examination revealed neither goiter nor signs of orbitopathy, and there was no personal or family history of thyroid disorders. His usual medications were hydroxyzine, zopiclone, famotidine, and clonazepam. The patient's clinical condition could not explain the high FT4 results with TSH within the reference range, which opened the possibility to consider unusual clinical situations.Initial results were confirmed in a new sample, so his endocrinologist consulted the biochemistry laboratory because of this lack of consistency with clinical presentation. Analytical records revealed similar results in 2016. Additionally, the patient provided analytical results from an outside laboratory, performed 2 months earlier using a different test method (Table 1, Lab 2),