Effects of calorie intake and sampling time on thyroid stimulating hormone concentration

BACKGROUND: This study aimed to investigate the effects of blood sampling after calorie intake on thyroid stimulating hormone (TSH) level, compared with blood sampling in fasting state. METHODS: This was a prospective, randomized, controlled study. Subjects from the outpatients in the department of endocrinology without evidence of thyroid diseases were included and then randomized into the fasting group, diet intake group, and glucose intake group, respectively. Fasting blood was collected from all subjects at 7:00 am for the measurement of TSH and free thyroxine (FT(4)) concentrations. Afterwards, the subjects were maintained at fasting state (fasting group), had an intake of the mixed diet with 400 kcal calories (diet intake group), and had an intake of 75 g glucose (glucose intake group), respectively, and blood was collected again 2 h later (9:00 am on the same day) for TSH and FT(4) level measurement and comparison. RESULTS: A total of 150 subjects were enrolled, of whom 146 met the inclusion criteria, with 48, 48, and 50 in the diet intake group, glucose intake group, and fasting group, respectively. The TSH in the diet intake group was significantly lower at 9:00 am (TSH(9am)) than the level at 7:00 am (TSH(7am)) (P < 0.001), with a median variation of -0.71 mU/L, and a median variation rate of -32.4%. In the glucose intake group, TSH(9am) was also significantly lower than TSH(7am) (P < 0.001), with a median variation of -0.73 mU/L, and a median variation rate of -31.5%. For the fasting group, TSH(9am) decreased slightly but was significantly lower than TSH(7am) (P < 0.001), with a median variation of -0.1 mU/L, and a median variation rate of -5.2%. According to TSH(7am) measurements, 9 subjects (3 subjects in each group) met the diagnostic criteria of subclinical hypothyroidism. However, according toTSH(9am) measurements, only 2 patients in the fasting group met the diagnostic criteria of subclinical hypothyroidism. CONCLUSION: Compared with the fasting state, the TSH level at 2 h after the calorie intake was decreased by about 30%, which might influence the diagnosis of subclinical hypothyroidism. TRIAL REGISTRATION: ChiCTR2100047454 (18/06/2021).