Many internal medical conditions can present with similar symptoms, which poses a diagnostic challenge for both patients and physicians. Two such conditions are iron deficiency and hypothyroidism. Both may manifest with comparable symptoms, such as chronic fatigue, cold intolerance, generalized weakness, or difficulties with concentration. As a result, many individuals who experience these symptoms mistakenly associate them with thyroid dysfunction, when the underlying cause may actually be iron deficiency—or vice versa. Why does this occur? Read the LuMedic article to learn more.

To Begin With – What Is Iron Deficiency?

Iron is an essential trace element required for the production of hemoglobin, the oxygen-transporting protein in red blood cells, as well as for the activity of numerous metabolic enzymes. Iron deficiency can impair hemoglobin synthesis and lead to the development of iron deficiency anemia. Importantly, symptoms may appear even before anemia becomes clinically evident.

The most common symptoms include:

– fatigue,
– physical and mental weakness,
– pale skin,
– dizziness,
– shortness of breath,
– palpitations,
– brittle nails,
– hair loss,
– impaired concentration.

It is important to emphasize that iron deficiency is widespread globally, particularly among women of reproductive age, pregnant individuals, and people experiencing chronic blood loss or inadequate dietary iron intake.

What Is Hypothyroidism?

Hypothyroidism is a condition in which the thyroid gland produces insufficient amounts of thyroid hormones (thyroxine – T4 and triiodothyronine – T3). As a consequence, the body’s metabolic processes slow down. Endocrinologists typically diagnose hypothyroidism based on elevated TSH levels accompanied by low free T4 concentrations in blood tests.

The most common symptoms of hypothyroidism include:

– fatigue,
– excessive sleepiness,
– cold intolerance,
– dry and pale skin,
– hair loss,
– constipation,
– weight gain,
– low mood.

Importantly, these symptoms often develop gradually and may initially be subtle, which unfortunately contributes to diagnostic difficulties.

Why Can the Symptoms Be Confusing?

There are several reasons why the clinical manifestations of iron deficiency and hypothyroidism may overlap:

Similar clinical presentation
Both conditions may lead to chronic fatigue, weakness, sensitivity to cold, pale skin, difficulty concentrating, and mood disturbances. Because thyroid disorders are widely discussed, patients often interpret these symptoms primarily as signs of thyroid dysfunction.

Lack of specific symptoms
Symptoms such as fatigue or generalized weakness are nonspecific and may result from numerous causes, including stress, nutritional deficiencies, chronic diseases, or hormonal changes. Their broad occurrence makes them easy to attribute incorrectly to a more commonly recognized condition such as hypothyroidism.

Coexistence of both conditions
In some individuals, both disorders may occur simultaneously. Hypothyroidism can impair iron absorption in the gastrointestinal tract, partly due to reduced gastric acid secretion. Conversely, iron deficiency may influence the synthesis of thyroid hormones and metabolic processes associated with their function.

The role of iron in hormonal metabolism
 Iron is required for the activity of enzymes involved in the biosynthesis of thyroid hormones. Iron deficiency may reduce the activity of these enzymes, potentially contributing to decreased thyroid hormone production.

Why Can Diagnosis Be Challenging?

One of the key issues is that standard blood tests—such as TSH measurement and complete blood count (CBC)—may not always detect subtle iron deficiencies or early stages of thyroid dysfunction.

For example, the complete blood count may remain within normal limits while ferritin, a marker of iron stores, is already reduced. For a more comprehensive assessment of thyroid function, in addition to TSH, it is often advisable to evaluate free T4 (fT4), free T3 (fT3), anti-thyroglobulin antibodies (ATG), anti-thyroid peroxidase antibodies (ATPO), and perform thyroid ultrasound examination.

Moreover, in patients already receiving thyroid hormone replacement therapy (e.g., levothyroxine), symptoms such as fatigue may persist despite normal thyroid hormone levels. In such cases, further diagnostic evaluation should include consideration of iron deficiency as a potential contributing factor.

Consequences of Misinterpretation

Incorrect interpretation of symptoms may lead to delayed diagnosis and treatment. Individuals with iron deficiency may respond poorly to treatment for hypothyroidism if the underlying iron deficiency remains unrecognized. Conversely, patients with undiagnosed hypothyroidism who receive only iron supplementation—without appropriate L-thyroxine therapy—may experience little or no improvement.

For this reason, comprehensive diagnostic evaluation is essential to distinguish between these two conditions and ensure appropriate treatment.