CAUTION: oral iron reduces levothyroxine absorption. Separate doses by at least 2 hours (UK SmPC); NICE and BNF guidance commonly advises at least 4 hours.
Mechanism
Iron salts form a chelate with levothyroxine in the gut, reducing absorption and potentially undertreating hypothyroidism. The effect is bidirectional but the clinically important direction is the loss of thyroid replacement.
Practical advice
Take levothyroxine on rising, on an empty stomach, with water. Take any iron supplement at least 4 hours later in the day. The same separation rule applies to multivitamins containing iron. If your TSH is rising despite stable levothyroxine dose, ask whether timing relative to iron is contributing.