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Background: Severe illness induces large changes in thyroid hormone economy. Non-thyroidal illness syndrome (NTIS), also known as low T3 syndrome, refers to thyroid hormone alterations induced by systemic or consumptive non-thyroidal diseases. These alterations strongly correlate with disease severity but are often overlooked in clinical practice.
Methods: Here we report a case of septic shock complicated by abnormal thyroid hormone (TH) levels. The patient presented with markedly reduced triiodothyronine (T3) and free triiodothyronine (fT3), profoundly suppressed thyroid-stimulating hormone (TSH), and elevated free thyroxine (fT4) levels - a rare biochemical profile in clinical settings. Laboratory confirmation of significantly elevated rT3 levels established the diagnosis of low T3 syndrome. The transient fT4 elevation likely results from profound TSH suppression combined with sepsis-induced inflammatory cytokines, which directly inhibit D2 activity, alongside reduced thyroxine-binding globulin (TBG) levels during the acute phase. The patient's TH levels normalized following resolution of sepsis symptoms.
Results: The patient exhibited markedly reduced T3 and fT3 levels with profoundly suppressed TSH and elevated rT3, consistent with NTIS rather than primary hypothyroidism. Transiently elevated fT4 may manifest in NTIS.
Conclusions: We recommend that when detecting reduced T3 with suppressed TSH in septic patients, clinicians should consider NTIS as a potential diagnosis. Measurement of rT3 and serial monitoring of TH levels are advised to avoid inappropriate therapeutic interventions.
DOI: 10.7754/Clin.Lab.2025.250410
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