Hypothyroidism is often responsible for hair loss. So is low ferritin, the storage form of iron. One of the main reasons why ferritin can decrease involves overgrowth of iron-stealing organisms, like candida. But hypothyroidism itself is also  associated with low serum ferritin levels. There are several reasons for this.

The relationship between iron and thyroid function is a bit of a catch-22. Low iron can perpetuate hypothyroidism, and hypothyroidism can perpetuate low iron levels.

How Hypothyroidism Can Cause Low Iron

Thyroid function controls your body’s metabolism; when it is low, digestion slows down, which is part of the reason why a classic symptom of hypothyroidism is constipation. Hypothyroidism also decreases hydrochloric acid (HCl) production, which means you won’t absorb your food very well. Iron absorption is one of the nutrients that will suffer most from low HCl.

Another common symptom of hypothyroidism for women is very heavy periods—which can lead to low iron due to blood loss.

How Low Iron Can Cause Hypothyroidism

This study shows that at least in rats, iron deficiency lowers the activity of the thyroid peroxidase (TPO) enzyme, which is necessary for the first two steps in producing thyroid hormone. This is because the enzyme is iron dependent.

Low iron will also decrease the conversion of the T4 hormone to the active T3 hormone, and increase conversion to the inactive reverse T3 hormone.

The Upshot

If your ferritin just won’t stay up without supplementation, consider that you might be subclinically hypothyroid—even if your levels on labs look “normal.”