Endocrine Abstracts

We report potential effects of antituberculosis (TB) treatment on vitamin D metabolism in the immediate postoperative period following total thyroidectomy.

A 50-year woman of Chinese origin was incidentally found to have papillary thyroid carcinoma with local nodal metastasis following FDG–PET imaging done to assess nature of lung nodules on chest X-ray and CT chest. The nature of lung lesion was uncertain and commenced on empirical TB treatment (four drug regimen). At this stage 25-hydroxy vitamin D (25-OHD) level was 71 nmol/l and calcium 2.20 (ref. range 2.1–2.55 mmol/l). One month later she had total thyroidectomy and level six neck dissection. She developed severe symptomatic hypocalcaemia in the immediate postoperative period with calcium 1.70. Symptoms improved with emergency treatment but remained symptomatic for 4 days requiring continuous calcium infusion. During this time she was commenced on alphacalcidol and oral calcium, and her calcium level stabilised by day 5. PTH level was 0.1 (ref. range 1.6–6.9 pmol/l) at discharge and improved to 0.4 within 2 weeks, while her calcium remained stable and was able to reduce dose of alfacalcidol.

TB treatment, especially rifampicin is reported to cause significant reduction in 25-OHD within few weeks after starting treatment. Several reports in the literature suggest adverse effects of TB treatment on vitamin D metabolism. It is reported that rifampicin binds to steroid and xenobiotic receptor (SXR) which mediates 24-hydroxylase activity leading to increased clearance of vitamin D (1).

We suggest that treating clinicians are aware of the need for closer monitoring of patients on TB treatment undergoing thyroid surgery and perhaps consider monitoring and correction of vitamin D insufficiency preoperatively to avoid complications of severe hypocalcaemia in the immediate postoperative period.

Reference: 1. Zhou C et al. Steroid and xenobiotic receptor and vitamin D receptor crosstalk mediates CYP24 expression and drug-induced osteomalacia. JCI 2006 116 (6) 1703–1712.