Anemia is defined by a reduced number of red blood cells or a lower-than-normal concentration of hemoglobin, the protein responsible for carrying oxygen throughout the body. Hypothyroidism, in contrast, results from an underactive thyroid gland that does not produce sufficient thyroid hormones (T3 and T4) to regulate metabolism. While the primary cause-and-effect relationship generally flows from low thyroid function to anemia, these two common conditions are frequently found together. They are intertwined through shared biological pathways, nutrient requirements, and overlapping symptoms that make diagnosis challenging. The answer to whether anemia can cause hypothyroidism is usually no, but a severe nutrient deficiency that leads to anemia can certainly impair thyroid function.
Shared Symptoms and Diagnostic Overlap
The frequent co-occurrence of anemia and hypothyroidism is often first noticed because the symptoms mirror each other. Both conditions are known to cause extreme fatigue, generalized weakness, an intolerance to cold temperatures, and cognitive issues often described as “brain fog.” These nonspecific complaints are common in primary care settings and can easily be attributed to either a lack of thyroid hormone or a lack of oxygen delivery due to anemia.
This overlap can sometimes lead to a delayed or incorrect diagnosis, as a patient’s symptoms may improve slightly with treatment for only one condition, masking the underlying second issue. For instance, a patient with iron deficiency anemia might also have undiagnosed subclinical hypothyroidism, and treating only the anemia might not fully resolve the fatigue. Because of this diagnostic challenge, healthcare providers often order comprehensive blood work, including thyroid function tests and a complete blood count, simultaneously to assess both systems. Identifying both conditions early is important for creating a complete and effective treatment plan.
How Anemia-Related Deficiencies Affect Thyroid Function
Although hypothyroidism is the more common cause of anemia, a severe nutritional deficiency that causes anemia, specifically iron deficiency, can directly hinder the thyroid gland’s function. Iron is a necessary component for the enzyme thyroid peroxidase (TPO), which is instrumental in the production of thyroid hormones. TPO’s primary role is to add iodine to the thyroglobulin protein, a step required to create the storage hormone thyroxine (T4) and the active hormone triiodothyronine (T3).
When iron levels are severely low, the activity of the TPO enzyme is reduced, which effectively slows down the thyroid’s ability to synthesize its hormones. Iron is also needed for the 5′-deiodinase enzymes that convert the less active T4 into the highly potent T3 in peripheral tissues. A lack of iron can impair this conversion process, potentially leading to or worsening subclinical hypothyroidism. Correcting the iron deficiency in these cases can sometimes help normalize thyroid hormone levels. Iron deficiency has also been associated with an increased presence of anti-thyroid antibodies, suggesting a link to autoimmune thyroid disease.
The Clinical Reality: When Hypothyroidism Causes Anemia
The most established clinical relationship is the reverse, where low thyroid hormone levels lead to the development of anemia through several distinct mechanisms. Thyroid hormones play a direct part in stimulating the production of red blood cells (erythropoiesis) in the bone marrow. Low levels of T3 and T4 slow the multiplication and maturation of red blood cell precursors, often resulting in a mild, normocytic-normochromic anemia, where the red blood cells appear normal in size and color but are simply fewer in number.
Hypothyroidism can also cause anemia by affecting nutrient absorption within the gastrointestinal tract. Reduced thyroid hormone levels can decrease the production of stomach acid, which is necessary for the proper absorption of Vitamin B12 and folate. Impaired B12 absorption can lead to a type of anemia called megaloblastic anemia, where the red blood cells are abnormally large. This is especially common in patients with Hashimoto’s thyroiditis, an autoimmune cause of hypothyroidism, because it frequently coexists with pernicious anemia, another autoimmune condition that attacks the cells needed for B12 absorption.
The third major cause relates to the effects of hypothyroidism on the menstrual cycle in pre-menopausal women. Low thyroid function can disrupt the normal hormonal balance, leading to heavy or prolonged menstrual bleeding, a condition known as menorrhagia. This chronic, excessive blood loss depletes the body’s iron stores over time, resulting in the development of iron deficiency anemia, which is characterized by small, pale red blood cells (microcytic, hypochromic anemia).
Coordinated Diagnosis and Treatment
When both anemia and hypothyroidism are suspected, a coordinated diagnostic approach is necessary to determine the root cause and the specific type of anemia present. Comprehensive blood testing typically includes a Complete Blood Count (CBC) to identify the size and number of red blood cells, Thyroid-Stimulating Hormone (TSH) and free T4 levels to assess thyroid function, and tests for iron stores (ferritin) and Vitamin B12 and folate levels. This panel helps the physician understand the exact relationship between the two conditions.
The treatment hierarchy usually prioritizes thyroid hormone replacement with a synthetic hormone like levothyroxine. Correcting the thyroid hormone deficit often resolves the anemia that is caused by reduced red blood cell production or B12 malabsorption. However, in cases of confirmed iron deficiency anemia, iron supplementation is also required, but the timing is important. Iron supplements can interfere with the absorption of levothyroxine, so patients are typically advised to take the medications several hours apart. The combination of thyroid hormone replacement and targeted nutrient supplementation has been shown to be more effective than treating either condition in isolation.