Endocrine Abstracts

A 41 year old lady presented to emergency department with tremors and palpitations. Her examination and routine bloods were normal but ECG showed sinus tachycardia (heart rate = 141 bpm). She was treated with propranolol and discharged. She was readmitted in emergency department with ongoing palpitations, lethargy and tremors. On examination, she had a diffuse goitre but no signs of Graves’ eye disease. The blood tests showed overt thyrotoxicosis (TSH<0.01 mU/l, FT4 = 49.1 pmol/l, FT3 = 27.7 pmol/l) with positive anti TSH receptor antibodies (Anti-TRAb levels = 11.3 IU/l) and a thyroid ultrasound revealed a diffusely enlarged goitre. She was started on treatment for Graves’ disease with carbimazole 15 mg BD along with propranolol. She was on 8 weekly endocrine follow up to monitor thyroid function tests (TFT). Within 4 months, her weight increased from 86.5 kg to 93.5 kg. She had the anticipated mild iatrogenic subclinical hypothyroidism (TSH = 7.35 mU/l, FT4 = 5.8 pmol/l, FT3 = 5 pmol/l). The dose of carbimazole was reduced to 5 mg once daily. Six months later, there was mild relapse of Graves’ disease (TSH<0.01 mU/l, T4 = 24.2 pmol/l, T3 = 9.2 pmol/l). Carbimazole was increased from 5 mg to 20 mg again. Her thyrotoxicosis went into remission (TSH = 1.56 mU/l, T4 = 9.9 pmol/l) 11 months after initial diagnosis, but the anti-TRAb was still positive (3.7 IU/l). Carbimazole was continued at 5 mg. Her TFTs started to show an unusual pattern of remission with normal TSH but raised FT4 and FT3 whilst being on carbimazole 5 mg.

TSH = 2.05 mU/l, T4 = 19.1 pmol/l and T3 = 8.1 pmol/l

TSH = 1.08 mU/l, T4 = 26.9 pmol/l and T3 = 14 pmol/l

This was discussed with the biochemist as it wasn’t the common pattern of early relapse with fully suppressed TSH(<0.01 mU/l). Hence it was thought to be an interference with biotin. On direct enquiry, the patient confirmed taking a variety of over the counter supplements containing biotin to help improve her hair and nail growth. This correlated well with the timing of unusual pattern of TFT. The carbimazole was stopped and her TFT on repeat check were completely normalised. Graves’ disease has a relapsing remitting course. It is not unusual to make frequent changes of carbimazole dosages based on TFT results every 6–8 weeks. The usual duration of treatment of Graves’ disease is 12–18 months to achieve remission and normalization of Anti-TRAb. Biotin has been reported to interfere with thyroid hormone assays and also give falsely elevated anti-TRAb levels. This can lead to inadvertent use of carbimazole in a patient with Graves’ disease in remission. This case highlights the importance of identifying use of supplements like biotin which can interfere with TFT results.