Short CommunicationThe impact of time of sample collection on the measurement of thyroid stimulating hormone values in the serum
Introduction
Thyroid stimulating hormone (TSH), or thyrotropin is a 28- to 30-kDa glycoprotein synthesized and secreted from basophile cells of the anterior pituitary gland. It is a member of the glycoprotein hormone family that includes follicle-stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin hormone (hCG). The largest and most complicated endocrine ligands known to date are glycoprotein hormones. hTSH has two noncovalently linked subunits, α-subunit (92 amino acids; common for other human glycoprotein hormones) and TSH β-subunit, the β-subunit carrier of the TSH-specific immunological and biological information [1], [2]. The genes located on chromosomes 6 and 1 encode human α-subunit and TSH β-subunit [3].
The central regulating mechanism for the biological action of thyroid hormones is the release of thyrotropin (TSH) from hypophysis. The thyroid stimulating hormone in serum varies to a great extent, the components of variation include preanalytical, analytical, and biologic variation [4].
Circulating TSH shows a normal circadian rhythm with a peak between 11:00 p.m. and 5:00 a.m. and a nadir between 5:00 p.m. and 8:00 p.m. [5]. Also secretory pulses occur every 2–3 h and are interspersed with periods of tonic non-pulsatile TSH secretion [5]. The low amplitude of the pulses and the long half-life of TSH result in mild circulatory variations [6]. But total daily amplitude of TSH over 24 hour period is approximately double [4]. Thus the clinical implication of such variation is that a patient may be designated as having subclinical hypothyroidism. Hence uniformity in testing under standard conditions is necessary.
Taking blood samples for the measurement of serum TSH is commonly performed in laboratories in the morning (7:00–10:00 a.m.), but sometimes in the afternoon too, with no recommendation that patients should be fasting. The aim of our research is to determine whether the time of blood sampling and fasting of patients have an impact on TSH values.
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Materials and methods
A total of 198 participants were enrolled in this study and classified into five groups. Group A (n = 35) had their first samples collection (phlebotomy) for TSH measurement between 7:00 and 8:00 a.m. at fasting and the second one after 140 min without food intake, i.e., again fasting. Group B (n = 56) had their first samples collection between 7:00 and 8:00 a.m. at fasting and the second one after 140 min with food intake. These subjects had no restrictions on the type and amount of food. Group C (n =
Results
Clinical data shows that the majority of the subjects were female (90%), aged 46.00 ± 9.10 years, except for group C with subjects aged 39.00 ± 9.10 years. There was no difference in age between the groups, except for group C. TSH values from the first samples collection and TSH values from the second samples collection in groups A, B, C, D and E are shown in Table 1. At baseline, there was no difference in TSH values between A and B groups, but after 140 min the difference was evident (p = 0.037). As
Discussion
Our study demonstrates a significant difference in serum TSH values of the same person, depending on the time of taking the blood for analysis. Study data shows that TSH in the early morning was higher compared to that in the later morning on the same day, taken from the same person. Previous studies by Scobbo et al. [7], Kamat et al. [8] and Bandhopadhyay et al. [9] show postprandial TSH decrease similar to that in our study. It is evident that the time of phlebotomy could have a significant
Conclusions
TSH values significantly vary between blood samples collected at different times of the day from the same person. Our results are strong evidence that the time of blood sample collection has an impact on TSH results. Sample collection time must be regulated for the purpose of TSH measurement standardisation and harmonisation.
Acknowledgment
We gratefully acknowledge the subjects who participated in our study for helping us with sample processing.
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