Does low thyroid function impair iron absorption by reducing gastric acid and slowing GI motility?

Thyroid hormones play a fundamental role in maintaining the physiological and chemical environment of the gastrointestinal tract. Research indicates that low thyroid function (hypothyroidism) creates a cascading effect that significantly interferes with nutrient processing, particularly the absorption of iron.

Clinical evidence and mechanisms

The relationship between thyroid function and the digestive environment is driven by two primary physiological changes:

• Impaired motility: Substantial evidence confirms that hypothyroidism slows gastrointestinal transit. Studies using radionuclide scintigraphy show that hypothyroid patients often have esophageal transit times more than double those of healthy individuals. Gastric emptying is also markedly delayed; research indicates only 23.6% of patients with subclinical hypothyroidism have normal gastric emptying, a condition that largely reverses (81% success rate) after levothyroxine replacement. This is driven by impaired neuromuscular transmission in the myenteric plexus and altered calcium handling in smooth muscle cells.
• Reduced gastric acid: Low thyroid function is strongly associated with hypochlorhydria (reduced stomach acid). This occurs through shared autoimmune pathways—such as the co-occurrence of Hashimoto's thyroiditis and autoimmune atrophic gastritis—and the systemic slowing of secretory functions. Low levels of gastric markers like Pepsinogen I correlate with high TSH, reflecting diminished acid output as thyroid function declines.

Impact on iron absorption

The reduction in gastric acid and motility creates a significant barrier to maintaining iron stores:

• Solubilization failure: Gastric acid is critical for lowering duodenal pH to approximately 2–3, which solubilizes dietary ferric iron (Fe³⁺) and allows its conversion to the absorbable ferrous form (Fe²⁺). Without sufficient acid, non-heme iron remains insoluble and cannot be transported via divalent metal transporter 1 (DMT1).
• Transit and mixing: Slowed motility limits the efficient mixing of food with what little acid is present and delays the delivery of iron to the proximal duodenum, the primary site of uptake. Over time, this chemical and mechanical dysfunction leads to clinically significant iron deficiency, a risk echoed by data showing a 2.56-fold increased risk of iron deficiency in patients using acid-suppressing medications.

Bottom line

Low thyroid function causes a physiological "slowing" of the digestive tract and a reduction in gastric acid secretion. These changes directly impair the solubilization and uptake of dietary iron, establishing a clear link between hypothyroidism and the development of iron deficiency.