Circadian and individual variability of TSH and thyroid hormones in subjects with euthyroidism

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To ivestigate circadian and individual variability of TSH, fТ4, fТ3 in subjects without evident thyroid dysfunction. 27 persons at the age of 18–60 years have been included. Measurements of serum TSH, fT4, fT3 were performed at 8.00–9.00 and 14.00–16.00 during the day and at 8.00–9.00 in 4–6 weeks. The median of TSH concentrations in the morning was 2.28 mU/l, at the daytime – 1.6 mU/l (р < 0.05). The amplitude of TSH circadian variability reached 58% (Me = 21.45%). According to the current TSH reference ranges (0.4–4.0 mU/l) all participants had an euthyroidism in the morning and at the daytime. According to the proposed TSH reference ranges (0.4–2.5 mU/l) 12 participants (44.4%) in the morning and 4 participants (14.8%) at the daytime have been classified as having a hypothyroidism. TSH levels in 4–6 weeks differed from initial on -42.8–7.71%. Statistically and clinically significant variability of fT4 has not been found. Variability of fТ3 has appeared statistically significant and correlation with TSH changes.

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