Endocrine Abstracts

Introduction: Achieving optimal levothyroxine replacement is more difficult in TSH deficiency compared to primary hypothyroidism because of the inability to be guided by TSH. A combination of clinical symptoms and free thyroxine levels (fT₄) are typically used to monitor replacement. We reviewed adequacy of levothyroxine replacement in our patients with pituitary disease, and compared with fT₄ levels in patients with primary thyroid disease.

Methods: We identified all 525 patients with a diagnosis of any type of pituitary tumour in our department’s clinical information system who had been seen in 2007 and 2008. A free T₄ (fT₄) value was found for 514 (97.9%). Two hundred and twelve were at high risk of TSH deficiency with macroadenoma and/or surgery and/or radiotherapy. We compared fT₄ values in patients with primary thyroid disease in our thyrotoxicosis shared-care scheme and thyroid register within the same timescale – assessing fT₄ only in samples with a normal serum TSH – 3777 samples euthyroid off treatment, 11 805 on levothyroxine and 5074 on carbimazole.

Results: fT₄ levels overall were lower in pituitary patients than in equivalent controls. Of high risk group not taking levothyroxine 17% had a free T₄≤11 pmol/l (10th centile for fT₄ in untreated controls) compared to only 8.4% of untreated controls. Furthermore, 38.9% of patients on levothyroxine had a free T₄≤13 pmol/l (10th centile for fT₄ in treated controls) compared to 9.5% of controls on levothyroxine with previous thyrotoxicosis and 13.4% of controls with primary hypothyroidism. Median fT₄ in controls on levothyroxine was 16 pmol/l and 20th–80th centile range was 14–19.

Conclusion: In our pituitary population we have found that levothyroxine doses are under-replaced compared to primary thyroid disease and that some untreated patients may actually be TSH deficient. The distribution of fT₄ in patients with primary thyroid disease on levothyroxine may guide optimum replacement levels.