What Ferritin and TSH Check For
Ferritin measures your body's iron stores and is the primary test for diagnosing iron deficiency, while TSH (thyroid-stimulating hormone) assesses thyroid gland function and is the first-line test for detecting thyroid disorders including hypothyroidism and hyperthyroidism.
Ferritin Assessment
Ferritin is an intracellular protein that stores iron and reflects total body iron stores. In the absence of inflammation, ferritin is the best performing test for diagnosing iron deficiency, with a goal level of 50 ng/mL regardless of biological sex 1.
Key Clinical Points for Ferritin:
- Diagnostic threshold: Ferritin <30 µg/L indicates iron deficiency in most contexts
- Optimal target: 50 ng/mL for iron repletion 1
- Critical limitation: Ferritin is an acute phase reactant, meaning it rises during inflammation, infection, or chronic disease, which can mask underlying iron deficiency 1
When Ferritin Results Are Unreliable:
- During active inflammation or infection
- In patients with chronic diseases (inflammatory bowel disease, autoimmune conditions)
- Within 4 weeks of intravenous iron administration 1
- In these situations, transferrin saturation (TSAT) <20% has higher sensitivity for detecting iron deficiency 1
TSH Assessment
TSH is secreted by the pituitary gland and regulates thyroid hormone production. It is the single most sensitive screening test for thyroid dysfunction 2.
What TSH Detects:
- Elevated TSH: Indicates hypothyroidism (underactive thyroid)
- Suppressed TSH: Indicates hyperthyroidism (overactive thyroid)
- Normal range: Typically 0.4-4.0 mIU/L, though reference ranges vary by laboratory and trimester in pregnancy
High-Risk Groups Requiring TSH Testing 2:
- Women over 60 years
- Patients with previous thyroid dysfunction or surgery
- Those with type 1 diabetes or autoimmune disease
- Family history of thyroid disease
- Symptoms suggestive of thyroid dysfunction
- Atrial fibrillation
The Ferritin-TSH Connection
Iron deficiency directly impairs thyroid hormone metabolism because iron is essential for thyroid peroxidase enzyme activity, which is required for thyroid hormone synthesis 3, 4. Recent evidence shows:
- Lower ferritin levels correlate with higher TSH levels 5, 6, 7
- Iron deficiency can elevate TSH and reduce free T4 6, 4
- Hemoglobin levels show stronger correlation with thyroid function than ferritin alone 4
- In iron-deficient states, the T3/T4 ratio increases as a compensatory mechanism 3
Clinical Implications:
When evaluating thyroid dysfunction, particularly in women of childbearing age or pregnant patients, concurrent iron deficiency should be assessed and corrected, as it may contribute to or worsen thyroid dysfunction 6, 7. The relationship is bidirectional—hypothyroidism, especially Hashimoto's thyroiditis, increases risk of iron deficiency 7.
Common Pitfalls to Avoid
- Don't interpret ferritin in isolation during acute illness: Use TSAT or soluble transferrin receptor instead 1
- Don't check iron parameters within 4 weeks of IV iron: Results will be spuriously elevated 1
- Don't screen general population for thyroid disease: Focus on high-risk groups and symptomatic patients 2
- Don't ignore iron status when managing thyroid disorders: Iron deficiency impairs thyroid hormone synthesis and should be corrected concurrently 3, 6, 4