Background: Twin sisters with infantile epilepsy developed biochemical hyperthyroidism after commencing Topiramate, which resolved post cessation. This case-report describes the events and has found limited evidence in the literature.
Case presentation: MCDA twins, 9 months of age, were admitted with seizure activity. Additionally twin 2, had neurocutaneous melanosis and a viral respiratory illness. Medications on admission were: Levetiracetam, Vigabatrin, Pyridoxine, Thiamine and Biotin. Both twins were commenced on Topiramate: 1 mg/kg per day in divided doses to optimize seizure control; no radiation or iodine exposure occurred within this time-period. Two days later thyroid levels were incidentally checked; both were biochemically hyperthyroid; but asymptomatic. Results showed: Twin 1 TSH: 0.11 mIU/l (0.274.2), FT4: 50.8 pmol/l (1023) with FT3: 8 pmol/l (3.16.8) and twin 2 TSH: 0.10 miu/l, FT4: 42 pmol/l with FT3: 6.8 pmol/l. Notably twin 1 was tested a month prior (Pre-Topiramate) and had raised TSH levels (8.94 miu/l), with normal FT4 (18.7 pmol/l); twin 2 had a normal TSH (1.94 miu/l) checked 3 days prior to commencing Topiramate.
Outcome: Following this, Topiramate was stopped and alternative medication started. One week post cessation, thyroid function had normalised. Twin 1 TSH: 0.71 miu/l, FT4: 13.2 pmol/l and twin 2 TSH: 3.8 miu/l and FT4: 12.3 pmol/l. It was considered unethical to re-challenge them with Topiramate.
Discussion: No peer-reviewed reports and one case report about hyperthyroidism secondary to Topiramate were found. Topirimate causes weight loss, although purported mechanism(s) arent fully known. Equally, there are numerous patients on Topiramate who havent developed deranged thyroid function. Further consideration is needed to elucidate the underlying cause; be it: drug interaction, metabolism related or other. Biotin is known to cause assay interference resulting in elevated FT4 and suppressed TSH. Although as both Twins were on Biotin throughout, including post cessation of Topiramate and normalisation of thyroid function, it doesnt appear to be the underlying cause here.
Learning Points: Low threshold to check thyroid function tests post starting Topiramate, even when patients are asymptomatic and awareness of possible drug/assay interactions resulting in thyroid dysfunction.