The use of thyroid function tests in the diagnosis of hypopituitarism: definition and evaluation of the TSH Index.
AffiliationChristie Hospital, Department of Endocrinology, Manchester, United Kingdom.
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AbstractSUMMARY Background: TSH secretion in hypopituitary patients may be decreased due to TSH deficiency, but it remains also under feedback inhibition by fT4. We propose a TSH Index (TSHI) as 'fT4-adjusted TSH', that corrects for any physiological TSH suppression, in order to provide a true estimate of pituitary thyrotrope function and any pathological pituitary suppression. Methods: 9519 TFTs (Bayer Immuno-1(R)) in 4064 patients of our institution were examined, including 444 patients investigated for hypopituitarism. Based on the physiological log-linear relationship between fT4 and TSH, we estimated the amount of feedback-induced change in logTSH per change in fT4, which allowed the extrapolation of logTSH to a fixed fT4 of 0, defining the TSH Index. TSH Indices were compared with other measures of pituitary function. Results: Feedback inhibition was estimated to cause a 0.1345 decrease in logTSH[mU/L] per 1pmol/L increase in fT4 concentration, i.e. TSHI=logTSH+0.1345xfT4. Patients with lower peak stimulated GH and cortisol concentrations had significantly lower TSHI (p<0.0001). TSH Indices measured before pituitary stimulation tests predicted highly significantly the risk of test failure (p=0.0002). 21.9% of all potential fT4-TSH combinations within the current reference ranges were identified as abnormal on the basis of the TSHI. Conclusion: The TSH Index provides an accurate estimate of the severity of pituitary dysfunction in hypopituitary patients based on simple TFTs. It predicts the probability of pituitary stimulation test failure, and extends the diagnosis of TSH deficiency into areas of the normal TFT reference ranges.
CitationThe use of thyroid function tests in the diagnosis of hypopituitarism: definition and evaluation of the TSH Index. 2009: Clin. Endocrinol.