Background: TSH secretion in hypopituitary patients may be decreased due to TSH deficiency but it also remains under feedback inhibition by free thyroxine (fT4). We propose a TSH index (TSHI), as 'fT4-adjusted TSH', that corrects for any physiological TSH suppression, to provide a true estimate of pituitary thyrotroph function and any pathological pituitary suppression.
Methods: A total of 9519 thyroid function tests (TFTs) (Bayer Immuno-1) 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 log TSH per change in fT4, which allowed the extrapolation of log TSH to a fixed fT4 of 0, defining the TSHI. TSHIs were compared with other measures of pituitary function.
Results: Feedback inhibition was estimated to cause a 0.1345 decrease in log TSH (mU/l) for 1 pmol/l increase in fT4 concentration, therefore TSHI = log TSH + 0.1345 x fT4. Patients with lower peak-stimulated GH and cortisol concentrations had a significantly lower TSHI (P < 0.0001). TSHIs measured before pituitary stimulation tests predicted highly significantly the risk of test failure (P = 0.0002). Of all potential fT4-TSH combinations within the current reference ranges, 21.9% were identified as abnormal on the basis of the TSHI.
Conclusion: The TSHI 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.