Can low iron levels and hypothyroidism (despite normal Thyroid-Stimulating Hormone (TSH)) cause hair loss?

Low Iron Levels and Hypothyroidism as Causes of Hair Loss

Yes, low iron levels can cause hair loss in a patient with hypothyroidism, even when TSH is normal. Both conditions independently contribute to hair loss and may have a compounding effect when present together.

Iron Deficiency and Hair Loss

Iron deficiency is the world's most common nutritional deficiency and is a well-established cause of hair loss, particularly in women 1. Iron serves as a cofactor for ribonucleotide reductase, the rate-limiting enzyme in DNA synthesis, making it crucial for tissues with high cellular turnover like hair follicle matrix cells 1.

The relationship between iron levels and hair loss has been documented in several studies:

• Low serum ferritin (reflecting total iron storage) is associated with chronic diffuse telogen hair loss 1
• Iron deficiency may be an initiating factor in hair loss conditions 1
• In patients with ferritin levels <40 ng/mL, the mean telogen ratio (resting hair phase) is significantly higher than the anagen ratio (growth phase), indicating more hair in the shedding phase 2

Hypothyroidism and Hair Loss

Hypothyroidism is a common cause of hair loss, even when TSH levels appear normal 3:

• Hair loss is a classic symptom of hypothyroidism listed in clinical guidelines 3
• It typically presents alongside other symptoms like fatigue, weight gain, cold intolerance, and constipation 3
• Hair loss in hypothyroidism often follows a diffuse pattern and may affect the lateral third of eyebrows 4
• Importantly, some patients may have tissue hypothyroidism despite normal TSH levels due to peripheral conversion issues or thyroid hormone resistance 1

Combined Effect and Management Approach

When both conditions coexist:

1. Address iron deficiency first:

   • Confirm iron deficiency with serum ferritin testing (the primary indicator of iron status) 1
   • Consider iron supplementation if ferritin levels are low (<40 ng/mL), particularly in women 2
   • Monitor for improvement in hair loss symptoms after iron repletion

2. Evaluate thyroid function thoroughly:

   • Despite normal TSH, consider the possibility of tissue hypothyroidism if clinical symptoms persist 1, 3
   • Look for other hypothyroid symptoms (fatigue, weight gain, cold intolerance) 3
   • In confirmed hypothyroidism, levothyroxine therapy can significantly improve hair growth, as demonstrated in case reports 4

3. Monitor for improvement:

   • Hair regrowth may take 3-6 months after addressing underlying causes
   • Continued hair loss despite treatment warrants further investigation

Important Considerations and Pitfalls

• Gender differences: Studies show iron deficiency may affect hair loss differently in females versus males, with stronger associations in women 1
• Vegetarian diet: Vegetarians have higher risk of both iron and vitamin B12 deficiency, which can compound hair loss issues 5
• Avoid overtreatment: Excessive levothyroxine can cause hyperthyroidism, which paradoxically can also cause hair loss 6
• Consider other micronutrients: While addressing iron, evaluate other potential deficiencies like vitamin B12 that may contribute to hair loss 7

The evidence suggests that correcting both iron deficiency and optimizing thyroid function can lead to significant improvement in hair growth in patients experiencing hair loss with these underlying conditions.