Circadian and individual variability of TSH, fT4, fT3 in patients with subclinical hypothyroidism

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To investigate circadian and individual variability of TSH, fТ4, fТ3 in patients with subclinical hypothyroidism (SH).20 patients with earlier diagnosed SH at the age of 18–60 years have been included. Measurements of serum TSH, fT4, fT3 were performed at 8.00–9.00 h and 14.00–16.00 during the day and at 8.00–9.00 h in 4–6 weeks. The median of TSH concentrations in the morning was 5,83 mU/l, at the daytime – 3.79 mU/l (р < 0.05). The amplitude of TSH circadian variability reached 73% (Me – 28.5%). According to the current TSH reference ranges (0.4–4.0 mU/l) 95% participants in the morning and 50% participants at the day-time have been classified as having a hypothyroidism. According to the proposed TSH reference ranges (0.4–2.5 mU/l) 95% participants in the morning and 80% participants at the daytime have been classified as having a hypothyroidism. TSH levels in4–6 weeks differed from initial on (-66) – 139%. Statistically and clinically significant variability of fT4 has not beenfound. Variability of fТ3 has appeared statistically significant and correlation with TSH changes. Variability of TSH level in patients with SH is highest. Reduction of the clinical importance of TSH circadian rhythm can be promoted by narrowing TSH reference range at the daytime.

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