Introduction: Isolated hypothyroxinemia (IH) in pregnancy, characterized by low free thyroxine 4 (FT4) with normal thyroidstimulating hormone (TSH), has a reported prevalence of 1.3–8%. We present a diagnostically challenging case that initially mimicked central hypothyroidism before evolving into classic IH. Case: A 37-year-old G4P3 female at 22 + 3 weeks gestation with β-thalassemia trait presented with palpitations and tachycardia (150–184 bpm). A bedside echocardiogram showed volume depletion. Her heart rate improved to 108– 116 bpm following a 1.5 L normal saline bolus. Incidentally, thyroid function tests (TFTs) revealed both low FT4 6.04 pmol/L (7.86–14.41) and low TSH 0.009 mIU/L (0.38–5.33). Anti-thyroid peroxidase antibody was markedly elevated at 360 IU/mL (<35). At 24 + 4 weeks, TFTs showed persistently low FT4 (5.84 pmol/L) and TSH (0.022 mIU/L). Differential diagnoses included central hypothyroidism or assay interference. Tests were repeated using different platforms. Both the Beckman system (FT4: 5.73 pmol/L, TSH: 0.086 mIU/L) and Roche system (FT4: 8.83 pmol/L [12.0–22.0], TSH: 0.11 mIU/L [0.27–4.20]) confirmed low values, ruling out assay interference. Following endocrinology consultation and given her asymptomatic status, IH was considered most likely, and a plan for close monitoring was initiated. Serial follow-up demonstrated biochemical evolution. By 31 + 6 weeks, TFTs showed a low TSH (0.753 mIU/L) with an FT4 level (5.86 pmol/L) within the normal reference range, a pattern typical of IH in pregnancy. Repeat TFTs 6 weeks postpartum showed both TSH and FT4 within normal range. Conclusion This case illustrates a rare and unexpected biochemical progression of IH in pregnancy, initially presenting with a pattern indistinguishable from central hypothyroidism. It underscores that in an asymptomatic, fertile patient, IH can manifest with an atypical pattern early in gestation. The findings suggest that vigilant serial monitoring, rather than immediate extensive pituitary workup, may be a prudent initial approach in such scenarios, as the biochemical profile can normalize toward the classic IH pattern as pregnancy advances.
Female ; Pregnancy ; Thyrotropin