Does low ferritin limit thyroid-related energy metabolism?

Iron deficiency, reflected by low ferritin, severely constrains metabolic rate through a dual pathway: impairing thyroid hormone biology and disrupting mitochondrial oxidative phosphorylation.

Thyroid hormone biology

• Enzymatic impairment: Iron is a critical cofactor for thyroid peroxidase (TPO), a heme-dependent enzyme. When ferritin stores are depleted, TPO activity decreases by 33% to 56%, directly hindering the synthesis of thyroxine (T4) and triiodothyronine (T3).
• Conversion and receptor binding: Low iron impairs peripheral 5′-deiodinase activity, which prevents the conversion of inactive T4 to active T3. It also reduces the binding of T3 to its nuclear receptors, blunting downstream intracellular thyroid hormone signaling.

Mitochondrial bioenergetics

• Electron transport chain disruption: Iron is an essential component of iron-sulfur (Fe-S) clusters and heme cofactors within Complexes I–IV of the mitochondrial electron transport chain (ETC).
• ATP depletion: Starving the ETC of iron slows electron transfer, reduces oxygen consumption, and collapses the mitochondrial membrane potential. This can decrease oxygen-dependent ATP production by up to 74% in highly oxidative tissues.
• Metabolic reprogramming: To compensate for this bioenergetic failure, cells shift away from oxidative phosphorylation toward anaerobic glycolysis, structurally limiting the tissue-level thermogenesis and cellular energy expenditure normally driven by thyroid hormones.

Bottom line

• Low ferritin directly limits thyroid-related energy metabolism by reducing active T3 availability while simultaneously stalling the mitochondrial respiratory machinery required to execute thyroid-driven ATP production.