Key result
A 39-year-old woman exhibited falsely elevated free T4 levels and suppressed TSH after starting low-dose levothyroxine, leading to an incorrect diagnosis of endogenous hyperthyroidism.
Case Report (n=1)
This case highlights the potential for falsely elevated free T4 levels leading to misdiagnosis of hyperthyroidism and inappropriate treatment.
A 39-year-old woman presented to our department in July 2023 for a second opinion on a presumed diagnosis of hyperthyroidism. Four months before attending, she consulted her gynecologist to plan pregnancy and was recommended treatment with levothyroxine (50 μg/day) after a blood test that showed a thyroid-stimulating hormone (TSH) level of 4.66 mU/L, a value within the laboratory reference interval (RI) (0.38−5.33 mU/L), but near the upper limit. Shortly after starting treatment, she began to feel nervousness and tachycardia, and consulted the endocrinology department of another hospital, where she underwent a new blood test, using an Alinity i Analyzer (Abbott). This analysis showed low TSH values (0.02 mU/L; RI 0.35−5.00) and high free thyroxine (T4) (44.7 pmol/L; RI 9.01−19.05). It was interpreted that such a high value of free T4 could not be attributed to treatment with a low dose of levothyroxine, and the patient was diagnosed with endogenous hyperthyroidism. Levothyroxine was discontinued and antithyroid drugs were prescribed. A blood test performed at the same center 1 month later showed a TSH level of 6.37 mU/L (RI 0.35−5.00) and free T4 > 64.4 pmol/L (RI 9.01−19.05).
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Cabrera-Argany et al. (2025) conducted a case report in Falsely elevated free T4 levels (n=1). Levothyroxine was evaluated. A 39-year-old woman exhibited falsely elevated free T4 levels and suppressed TSH after starting low-dose levothyroxine, leading to an incorrect diagnosis of endogenous hyperthyroidism.
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