The objectives were to study whether a smaller area of irradiation resulted in a significant reduction in the dose of absorbed radiation to the thyroid gland for three radiographic modalities commonly used in cephalometric treatment planning. The aim of this research, was to study the effect of the field size on the absorbed dose of radiation to the thyroid gland, as measured with thermoluminescent dosimeters (TLD) in a phantom. Three radiographic modalities were used: panoramic, cephalometric, and intra-oral. For each modality, a large and a small collimator were used. A Rando Alderson phantom comprising a natural human cranium and cervical vertebrae embedded in a thermoplastic material simulating human soft tissue was used to measure absorbed dose to the thyroid gland. These measurements were made using lithium fluoride (LiF) TLDs. A significant reduction in absorbed dose to the thyroid gland was obtained by reducing the collimator in all three modalities. A 15% reduction in absorbed dose to the thyroid gland was achieved for the combination of small field sizes compared to the combination of large field sizes. Combining the use of selection criteria together with a smaller field of irradiation than normally used made a reduction in absorbed dose to the thyroid gland of around 30% possible.