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Objectives: Thyroid hemiagenesis (THA) is a congenital anomaly of unknown origin. The aim of this study was to assess for the first time whether there is any seasonality in the date of birth of patients with THA in comparison to general population. Methods: The studied group consisted of 105 patients with THA. The control data regarding total live births in Poland were derived from the Polish Statistical Annals (n = 2 421384). Both groups were subdivided according to the month of birth, season of birth and quarter of the year. Results: Analysis of the months of birth revealed that patients with THA were born significantly less often in October than control subjects (P = 0.0217). Although a trend is observed, that more patients with THA were born in the 1st quarter of the year and in winter, while THA patients were less often born in autumn or in 4th quarter of the year, the difference did not reach statistical significance. Conclusions: Month of birth may exert some effect on the risk of developing THA. Studies on larger cohorts are needed to confirm the observation. The results suggest a potential modifying effect of environmental factors in the pathogenesis of THA. [ABSTRACT FROM AUTHOR]