Thyroid Function Tests in Relapsing Fever

To the Editor.  —We report an unexpected disturbance of thyroid function in patients with a febrile illness of short duration. These patients showed a reduction in serum total thyroxine (TT₄) level accompanied by a fall in free thyroxine (FT₄) level; the thyroid-stimulating hormone (TSH) level remained unchanged. Clinical hypothyroidism was not detected.In ten patients (nine male, one female) with relapsing fever, blood samples were taken on admission and during convalescence (less than five days after the initial sample). The TT₄ level was measured by a double antibody radioimmunoassay; FT₄ and TSH levels were measured by direct radioimmunoassay (Amerlex) and immunoradiometric assay (Maia-clone) kits, respectively.The results were consistent with a "sick-euthyroid syndrome," which is characterized by a low serum total triiodothyronine level, normal TSH levels, and variable alterations in TT₄ and FT₄ levels in patients who are clinically euthyroid.₁ In previous studies