Circannual and within-individual variation of thyroid function tests in normal subjects

Area of Science:

• Endocrinology
• Clinical Chemistry
• Human Physiology

Background:

• Thyroid function tests are crucial for diagnosing thyroid disorders.
• Understanding individual variation and seasonal influences on thyroid hormones is essential for accurate interpretation.

Purpose of the Study:

• To assess within-individual variability of serum thyroid hormone concentrations over one year.
• To identify seasonal patterns in thyroid function tests.
• To evaluate the implications of individual variation for interpreting thyroid hormone results against population-based reference ranges.

Main Methods:

• Monthly blood sampling from normal subjects over 12 months.
• Determination of serum thyroxine and triiodothyronine concentrations.
• TSH (Thyroid-Stimulating Hormone) response testing to TRH (Thyrotropin-Releasing Hormone) every 3 months.

Main Results:

• Thyroid hormone concentrations were maintained within narrow limits for each individual.
• Within-individual variation constituted a small fraction of the overall population-based variation (reference interval).
• Seasonal variations were observed, with higher thyroxine and triiodothyronine levels in winter.
• A trend towards increased TSH response to TRH was noted in winter.

Conclusions:

• High degree of individuality in thyroid hormone levels suggests population-based 'normal ranges' may be misleading.
• Seasonal fluctuations in thyroid hormones and TSH response indicate potential central regulation by the hypothalamic-pituitary-thyroid axis in response to environmental temperature.
• Further research into seasonal effects and individual thyroid hormone profiles is warranted.