TRH testing, T4-thyrotoxicosis and the aging thyroid gland.

Secondary thyroid function tests were compared in 41 mildly thyrotoxic and 36 euthyroid patients with an elevated free thyroxine index (FT4I). A serum TSH measurement 20 minutes after intravenous TRH (delta TSH) most reliably separates these two groups. A significant delta TSH response (greater than 0.5 microU/ml) is also helpful in excluding clinical thyrotoxicosis in patients with nodular goitre. The free T3 index was normal in one-third of mildly thyrotoxic patients and in all euthyroid patients with a falsely elevated FT4I. Blunted delta TSH responses to TRH in elderly New Zealand women were associated with nodular goitre or occult thyroid nodularity revealed only by thyroid scan. The reduced TRH responses are more likely due to partial thyroid autonomy than reduced synthetic capacity of thyrotrophs in old age.