Significance of latent hyperthyroidism

Abstract

In euthyroidism the circadian rhythm and pulsatility of TSH is well known. With regard to hyperthyroidism only very preliminary data were described. In this study we investigated the secretion pattern of the pituitary-thyroid axis hormones during 24 h in latent and overt hyperthyroidism and in euthyroidism with regard to common and different properties. Blood was obtained for 24 h at 10-min intervals. In euthyroidism we found intraindividually three overlapping patterns of TSH, which are different in amplitude and frequency and can be found interindividually, too. These patterns are equal to the circadian rhythm, pulsatile secretion and lastly to the methodic rustle. The circadian rhythm in latent hyperthyroidism is distinctly suppressed and in overt hyperthyroidism totally. Whereas in latent hyperthyroidism pulsatile secretion is extant, in overt hyperthyroidism the TSH pulses are absent. To record the patients' TSH circadian rhythm with only three blood samples, we defined the TSH-Triplex. In young as well as in elderly healthy volunteers it demonstrated significantly higher TSH levels at midnight (at 24:00 h) than it did at 4 p.m. and 8 a.m. The present study shows a significantly different TSH pattern in latent hyperthyroidism compared to euthyroidism. It should be discussed whether latent hyperthyroidism could be defined as hyperthyroidism stage I. On the other hand, latent hyperthyroidism could be an illness with its own cause, different from hyperthyroidism. Our data suggest that the laboratory findings of latent hyperthyroidism in each age are non-physiological. However, the cause for this disorder is unclear until now; hence further investigations are necessary.