Iron & levothyroxine

Our assessment

Based on FDA adverse event data, levothyroxine carries a significant safety burden with 94,009 total adverse event reports received. The most important finding is that 45,961 of those reports were classified as serious, representing nearly half of all cases FDA FAERS ↗. This high proportion of serious outcomes underscores the need for careful monitoring and precise dosing, as even small deviations can lead to clinically meaningful effects.

Safety profile

FDA data shows specific adverse events reported for levothyroxine include: 1,830 reports of anxiety, 1,696 reports of headache, 1,665 reports of nausea, 1,198 reports of dizziness, 1,163 reports of fatigue, 1,093 reports of palpitations, 1,067 reports of dyspnea (difficulty breathing), 1,012 reports of alopecia (hair loss), 962 reports of insomnia, 859 reports of diarrhea, 852 reports of tremor, 840 reports of vomiting, 764 reports of asthenia (weakness), 751 reports of weight decreased, 674 reports of chest pain, 671 reports of pruritus (itching), 652 reports of hyperhidrosis (excessive sweating), 636 reports of tachycardia (rapid heart rate), 635 reports of arthralgia (joint pain), 614 reports of pyrexia (fever), 597 reports of pain in extremity, 580 reports of rash, 568 reports of myalgia (muscle pain), 557 reports of hypotension (low blood pressure), 554 reports of hypoesthesia (reduced sensation), 539 reports of hypertension (high blood pressure), 498 reports of malaise, 493 reports of increased appetite, 491 reports of feeling abnormal, 490 reports of back pain FDA FAERS ↗. These side effects range from minor (such as hair loss or itching) to serious (such as cardiac arrhythmias and breathing difficulty).

Safety profile

Patients who should be especially cautious include those with cardiovascular disease, as levothyroxine can exacerbate conditions like angina, arrhythmias, and hypertension. Older adults and patients with adrenal insufficiency require lower starting doses and slower titration to avoid adverse cardiac events. Drug interactions are significant: levothyroxine absorption is reduced by calcium carbonate, iron supplements, antacids, and bile acid sequestrants FDA FAERS ↗. Dose thresholds are critical — overtreatment (excessive dosing) can lead to thyrotoxicosis with symptoms including tachycardia, anxiety, weight loss, and heat intolerance, while undertreatment leaves hypothyroidism unmanaged FDA FAERS ↗. Data are not available for specific dose thresholds for all populations.

Our recommendation

Start levothyroxine at the lowest effective dose, typically 1.6 mcg/kg/day for healthy adults, and titrate based on TSH levels every 6-8 weeks. Monitor for signs of overmedication: palpitations, tremor, insomnia, and anxiety. For undermedication: fatigue, weight gain, and cold intolerance. Take levothyroxine on an empty stomach, 30-60 minutes before food or other medications, with water only FDA FAERS ↗. Always consult your healthcare provider before adjusting your dose.