Background: Despite relevant evidence that the absorption and/or action of L-thyroxine is compromised by various factors including the diet, until 2014 the British National Formula (BNF) recommended that L-thyroxine be taken preferably before breakfast only, with no further clarifications. To follow is one of the cases where untimely food intake in hypothyroid patients manifested with severe hypothyroidism due to L-thyroxine malabsorption.
Case: A 79 years old Caribbean male patient with background history of autoimmune hypothyroidism was admitted with poor oral dietary intake and dehydration. TSH on admission was 0.73 mU/l, reflecting adequate replacement with 125 micrograms of L-thyroxine OD. Hospital stay was prolonged for various reasons. Within 2 weeks from admission the patient developed severe, symptomatic hypothyroidism (fT4<5 pmol/l, TSH>100 mU/l). Apparently, L-thyroxine during hospital stay was given at 0800 h, just before breakfast. A simple change in the timing of administration of L-thyroxine to 0700 h, >30 min before breakfast and isolated from other tablets, rapidly improved both symptoms of hypothyroidism and thyroid function tests within days (fT4: 12.4 and 21.1 pmol/l after 5 and 14 days, respectively), without need for an increase in the replacement dose.
Discussion: In response to the evidence provided, which also included a systematic review of the literature, the BNF has changed the phrasing regarding the correct intake of L-thyroxine to preferably 30 min before food, other tablets and caffeine-containing beverages (implemented March 2014). This simple measure is likely to facilitate more reliable L-thyroxine absorption and reduces swings in thyroid function tests. With a hypothyroidism prevalence of at least 2% in the UK, reducing outpatient and GP appointments by 1 appointment/year only in every 5th patient results in estimated cost savings of £20 000 000 for the NHS.