Does low ferritin indicate depleted iron stores and affect thyroid hormone metabolism?

Low serum ferritin is a highly specific biomarker for depleted body iron stores, which play an essential, dual role in both the synthesis and peripheral metabolism of thyroid hormones.

Clinical assessment of iron stores

• Diagnostic utility of serum ferritin: In the absence of confounding inflammatory pathology, serum ferritin is the most reliable non-invasive marker of depleted iron reservoirs.
• Established diagnostic thresholds:
  • A threshold of <15 µg/L is highly specific (~98%) but fails to detect up to half of all clinically significant iron depletion.
  • A threshold of <30 µg/L is the optimal clinical standard, providing 97% to 99% specificity against bone marrow biopsy findings.
  • A threshold of <45 µg/L improves sensitivity to 85% (maintaining 92% specificity) for identifying depletion in anemic adults.

Mechanistic impact on thyroid metabolism

• Impaired hormone synthesis: Iron is a critical constituent of thyroid peroxidase (TPO), a heme-dependent enzyme responsible for iodide oxidation and the coupling reactions that produce thyroxine (T4) and triiodothyronine (T3). Iron deficiency directly decreases TPO activity, lowering overall hormone synthesis.
• Disrupted peripheral conversion: The production of active T3 relies heavily on the peripheral conversion of T4, a process mediated by type 1 and type 2 5′-deiodinases. Iron deficiency impairs hepatic and peripheral 5′-deiodinase activity through mitochondrial dysfunction and altered systemic metabolic signaling.
• Shift toward inactive pathways: This enzymatic block suppresses the generation of active T3 and shifts thyroid metabolism toward deactivating pathways, increasing the conversion of T4 into inactive reverse T3 (rT3).

Bottom line

• A serum ferritin level below 30 µg/L specifically indicates depleted iron stores, which directly compromises thyroid function by reducing heme-dependent TPO activity and impairing peripheral 5'-deiodinase conversion of T4 to active T3.