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 departments 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 20th80th centile range was 1419.
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.