Objective: To investigate the clinical significance of thyroid-stimulating hormone (TSH) circadian variability in patients with hypothyroidism.
Design: A total of 20 women with subclinical hypothyroidism and 22 patients taking L-thyroxine replacement therapy for hypothyroidism were enrolled in the study. Measurements of serum TSH levels were done twice a day from 08.00 to 09.00 a.m. and from 2.00 to 4.00 p.m.
Results: The morning median TSH value in the patients with subclinical hypothyroidism was 5.83 mU/L; in the afternoon, it was 3.79 mU/L. The range of TSH circadian variability reached the level of 73%. According to the current TSH reference interval, hypothyroidism was not diagnosed in about 50% of the cases in the afternoon. The morning median TSH value in the patients taking l-thyroxine was 3.27 mU/L; it decreased to the value of 2.18 mU/L in the afternoon. The range of TSH circadian variability reached the level of 64.7%. Further analysis demonstrated inadequate compensation of hypothyroidism, which was defined in 45.5% of the morning samples and in 9% of the afternoon samples (p < 0.05).
Conclusion: The time of blood sampling has an important role in the interpretation of TSH levels. Moreover, the high TSH circadian variability should be considered in discussions about the narrowing of its reference range.