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

Blood was taken from normal subjects at monthly intervals over a period of one year for subsequent determination of serum thyroid hormone concentrations. Thyroid-stimulating hormone (TSH) responses to TSH-releasing hormone were performed at 3-monthly intervals. This study provided data on within-individual variation and on seasonally-related changes of these thyroid function tests. The results showed that, within an individual, thyroid hormone concentrations are maintained within narrow limits. For both thyroxine and triiodothyronine the component contribution of within-individual variation to the population-based variation (the latter also termed the 'reference interval', or colloquially the 'normal range') was small. This high degree of individuality implies that rigorous comparison of thyroid hormone results against a population-based 'normal range' can be potentially misleading. Despite the limited within-individual variation, seasonally-related changes in thyroid hormone concentrations were apparent, with higher thyroxine and triiodothyronine values seen in winter months. A tendency to a greater TSH response to TSH-releasing hormone was also noted at this time. Conceivably these changes could reflect a centrally-mediated response of the hypothalamic-pituitary-thyroid axis to environmental temperature.