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 NICE CKS. This includes multivitamins and other over-the-counter medications NICE CKS.
To minimise interference, advise the patient to leave an interval of four hours between taking levothyroxine and any potentially interfering drug, if possible NICE CKS.
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 NICE CKS. If a drug interaction is identified as a contributing factor and is optimally managed, adjust the dose of levothyroxine therapy as appropriate NICE CKS.
The aim of treatment is to maintain TSH levels within the reference range NICE NG145. If symptoms persist, consider further adjusting the levothyroxine dose to achieve optimal wellbeing, while avoiding doses that cause TSH suppression or thyrotoxicosis NICE NG145. For adults, TSH should be measured every 3 months until stable (two similar measurements within the reference range 3 months apart), then annually NICE NG145.
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 NICE CKS.