Of all the nutrient deficiencies that intersect with Hashimoto’s, iron and ferritin are perhaps the most underdiagnosed — because their symptoms are almost identical to thyroid symptoms themselves, and because the test that matters (ferritin) is often not run.
Iron vs ferritin: what’s the difference?
Iron is the mineral itself. Ferritin is the protein that stores iron — it’s your iron reserve tank. You can have low ferritin while your hemoglobin (the standard anemia test) still reads normal, because your body defends hemoglobin by drawing down the stores first. This is why standard “anemia” tests miss iron deficiency at the stage that matters most for Hashimoto’s.
How iron and ferritin connect to thyroid function
Iron is required for the enzyme thyroid peroxidase (TPO) — the same enzyme your Hashimoto’s antibodies attack. Iron deficiency impairs thyroid hormone synthesis directly. It also impairs the conversion of T4 to T3. This creates a compounding effect: low ferritin weakens the thyroid’s ability to make and convert hormone, on top of the damage the antibodies are already causing (PMC).
Low ferritin as a standalone driver of symptoms
Here’s the crucial point: low ferritin causes its own symptoms that are indistinguishable from thyroid symptoms.
- Fatigue (often severe)
- Hair shedding (ferritin below 70 ng/mL is associated with diffuse hair loss in women)
- Brain fog and difficulty concentrating
- Breathlessness and poor exercise tolerance
- Cold intolerance
If you have all of the above, are on levothyroxine with “normal” TSH, and still feel terrible — a low ferritin could be contributing as much as (or more than) any thyroid gap. It’s worth testing.
What to test and what to aim for
Ask your doctor for a serum ferritin test (not just hemoglobin or full blood count). Many practitioners aiming for optimal thyroid health target ferritin above 70–100 ng/mL for symptomatic relief, while lab “normal” often starts at 12–15 ng/mL. The difference between “not anaemic” and “enough iron to support thyroid function and hair growth” can be enormous.