Background: Sensitivity is an important dosimetric issue particularly in personal dosimetry where the range of doses is often very low to medium. In-service aging is assumed to be of negligible effect on the response of thermoluminescent dosimeters (TLD), consequently, some service providers may mix fresh TLDs to in-use ones and simply normalize the readings of TLDs through batch calibration. This may lead to an increase in the coefficient of variation of the batch.Objective: The correlation between the sensitivity of LiF(Mg:Ti) and its in-service life is quantitatively studied in this paper. Method: Three groups of dosimeters with different service lives were chosen from the inventory of the national TLD service provider. The TLDs were then exposed to different doses and their TL responses were measured and compared. Result: By analyzing the results obtained from groups with different in-service ages, it was shown that the sensitivity of the dosimeters degrades with age in long term, and the amount of this sensitivity loss was measured. The reading standard deviation of the dosimeters degrades with the service life for low doses but seems to be insensitive to the service life in medium and high doses.Conclusion: A more efficient annealing approach is recommended time to time for TLDs. Moreover, the calibration dosimeters must be updated whenever a big difference between the sensitivity of new and used TLDs is seen.