Previously unsuspected hypopituitarism can be identified from suggestive thyroid function tests. The laboratory is in an excellent position to initiate further investigation when a low free thyroxine (fT4) and normal or marginally elevated thyroid-stimulating hormone (TSH) are noticed. Jostel et al. suggest that the use of a TSH index (TSHI) can further assist in providing a true estimate of pituitary thyrotroph function.

Based on the physiological log-linear relationship between fT4 and TSH, the authors calculated that feedback inhibition was estimated to cause a 0.1345 decrease in log TSH for a 1 pmol/L increase in fT4. TSHI was then defined as log TSH + 0.1345 × fT4, using results from the Bayer Immuno-1^® system. A reference population (n = 2008) was used to define a TSHI reference range and standardized TSHI values.

The accuracy of the index was evaluated against dynamic function testing in 444 patients, on whom glucagon stimulation tests or insulin tolerance tests had been performed. Lower peak hormone concentrations poststimulation were associated with significantly reduced TSHI values. Furthermore, a single TSHI obtained two weeks prior to the dynamic function testing predicted the risk of failing the stimulation test (P < 0.0002).

The authors state that 22% of all TSH–fT4 combinations where both analytes are within the reference range have abnormal TSHIs. Although we are not told the likely frequency of such combinations in the general population this seems a very high percentage when considering the incidence of hypopituitarism which is estimated at 4.2 cases/10,000 adult population per year. This excludes the use of this index in screening for hypopituitarism.

However, the TSHI does provide a numerical estimate of the severity of pituitary dysfunction in hypopituitary patients. It can be used to predict the probability of pituitary stimulation test failure, and its potential use in monitoring chronic pituitary disease warrants further study.