How should I adjust the levothyroxine dosage for a patient with hypothyroidism who is also taking other medications?

When a patient with hypothyroidism is taking other medications, it is important to consider potential drug interactions with levothyroxine (LT4) that may reduce its absorption or increase its requirements ¹. This includes multivitamins and other over-the-counter medications ¹.

To minimise interference, advise the patient to leave an interval of four hours between taking levothyroxine and any potentially interfering drug, if possible ¹.

If thyroid function tests (TFTs) remain abnormal or the patient has persistent symptoms despite adequate or escalating levothyroxine doses, assess for possible causes, including drug interactions ¹. If a drug interaction is identified as a contributing factor and is optimally managed, adjust the dose of levothyroxine therapy as appropriate ¹.

The aim of treatment is to maintain TSH levels within the reference range ². If symptoms persist, consider further adjusting the levothyroxine dose to achieve optimal wellbeing, while avoiding doses that cause TSH suppression or thyrotoxicosis ². For adults, TSH should be measured every 3 months until stable (two similar measurements within the reference range 3 months apart), then annually ².

Consider referral to an endocrinologist if the patient is on adequate or escalating levothyroxine doses, and their TSH level remains persistently raised after underlying causes, such as drug interactions, have been excluded or managed ¹.