This is a very unique case scenario where we are treating mild fatigue related to anemia, and a new prescription for iron creates more fatigue for our patient.
A 72-year-old female with a history of primary hypothyroidism, osteoporosis, hypertension, and iron deficiency anemia presents to her primary care provider complaining of worsening fatigue over the past 3 months. Her medications include levothyroxine 100 mcg daily, amlodipine 5 mg daily, and vitamin D3 2,000 units daily.
Three months earlier, laboratory testing revealed a hemoglobin of 11.2 g/dL (normal 12–16 g/dL) and ferritin of 18 ng/mL (normal 20–200 ng/mL), consistent with mild iron deficiency anemia. She was started on ferrous sulfate 325 mg daily. At the same visit, a calcium carbonate 600 mg twice daily supplement was added for bone health.
At today’s visit, she reports that despite taking the iron supplement consistently, her fatigue has worsened. She denies bleeding, weight loss, shortness of breath, or depressive symptoms. Repeat laboratory testing shows:
- Hemoglobin: 12.0 g/dL
- Ferritin: 42 ng/mL
- TSH: 24 mIU/L (normal 0.4–4.5 mIU/L)
- Free T4: Low
Upon further questioning, the patient explains that she takes all of her morning medications together with breakfast, including levothyroxine, calcium carbonate, and ferrous sulfate.
Both calcium carbonate and iron supplements can significantly decrease the gastrointestinal absorption of levothyroxine when taken at the same time (excellent board exam question). This is a very unique case scenario where we expect the anemia improvement to help fatigue, but adding iron creates more fatigue due to a drug interaction. Although the patient’s anemia improved, her hypothyroidism worsened, as evidenced by the markedly elevated TSH and low free T4. The resulting hypothyroidism is the most likely cause of her ongoing fatigue. Patients should generally separate levothyroxine from calcium and iron products by at least 4 hours and take levothyroxine on an empty stomach for optimal absorption. We need to make sure that patients are educated about this interaction and how to properly administer these medications.
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