Endocrine Abstracts

Introduction: IIH is associated with hypo and hyperthyroidism. We report a case of IIH where the likely precipitant was the treatment for hypothyroidism itself, levothyroxine, and the challenges this presents.

Case: A 14-year-old girl was diagnosed with primary hypothyroidism following investigations for short stature (height 137 cm < 0.4^th centile). Serum assay showed TSH 538 mU/l (0.55–4.78 mU/l), T4 0.68 pmol/l, (1.7–22.7 pmol/l), negative anti-thyroid antibodies. Thyroid ultrasound showed a small gland. Levothyroxine 50 micrograms once daily was started.

Four days after initiation of treatment she developed headaches. She represented one month later with blurred vision in the right eye. Serum assay showed TSH 22.64 mU/l, T4 12.4 pmol/l, thyroid peroxidase antibody 117 iu/ml. Fundoscopy revealed bilateral papilloedema, cotton wool spots and splinter haemorrhages. The blind spot was enlarged. MRI showed a small pituitary and magnetic resonance venography was normal.

Lumbar puncture was performed; the opening pressure was 40 cm H₂O the closing 15 cm H₂O, IIH was diagnosed. She was started on acetazolamide, her symptoms resolved within two weeks.

Conclusion: The literature reveals eleven case reports where IIH occurred with levothyroxine initiation. Of those six had a normal T4 at the time of IIH diagnosis.

The mechanism by which levothyroxine causes IIH is unclear. Hypothyroid patients have a reduced ability to excrete free water. We also know there is an association between hypothyroidism and hyponatraemia.

Levothyroxine normalises tissue composition and excess water is excreted. Could an excessive diuresis occur, along with normalisation of the hyponatremic state; leading to altered CSF dynamics and predispose to IIH? Our patient was profoundly hypothyroid; could the alterations in CSF dynamics be more marked in this patient? Perhaps the risk of IIH could be minimised by more gradual introduction of levothyroxine.

The presumed precipitant of IIH, levothyroxine, could not be stopped without the patient becoming hypothyroid. There was no evidence available for substituting T4 for T3 in this scenario. Indeed T3 can increase venous pressure, a proposed mechanism in the development of IIH. In this case giving acetazolamide resulted in an improvement in symptoms. However optic nerve fenestration preserves vision in resistant cases.