Thyrotropin Secretion in Healthy Subjects Is Robust and Independent of Age and Gender, and Only Weakly Dependent on Body Mass Index

Context: Studies of the influence of sex, age, and body weight on TSH secretion are not unanimous. Most reports are based on a single TSH measurement; studies using frequent blood sampling are scarce and include a limited number of selected subjects. Objective: The goal was to investigate TSH dynamics in 117 healthy adults. Methods: TSH was measured by a sensitive immunofluorometric assay. Secretion parameters were quantified by automated deconvolution, approximate entropy [ApEn], spikiness, and diurnal properties. Results: Mean age was 43 years (range, 22-77 y). Mean body mass index (BMI) was 26.8 kg/m(2) (range, 18.3-39.4 kg/m(2)). Daily TSH secretion was 45.4 mU/L (range, 8.0-207 mU/L). There were no sex differences in secretion parameters, including pulse frequency; basal, pulsatile, and total secretion; pulse mode; half life; pulse regularity; ApEn; spikiness; and nycthemeral properties. BMI was positively related to basal secretion. Total secretion correlated negatively with free T-4 (R = 0.225; P = .018). The onset of the nocturnal surge was delayed by increasing BMI and advanced by increasing age. ApEn and spikiness correlated positively with age, especially in men. The 9AM sample correlated strongly with the total 24-hour secretion, explaining two-thirds of the variability. Conclusion: This study shows that the 24-hour TSH secretion in healthy volunteers is stable and robust and not influenced by sex, BMI, and age. ApEn in the elderly, especially men, is increased, pointing to a less tight feedback control. Furthermore, aging is associated with advance shifting of the TSH rhythm, which is a phenomenon also observed in other biological rhythms