The Monte Carlo method can be used to simulate deterministic processes such as the interaction of nuclear particles with matter, using a probabilistic approach. For radiation transport, the method is realised using Monte Carlo radiation transport codes. Performing simulations often has a distinct advantage over experimental studies, especially when physical measurements would be impracticable or even impossible to undertake. The application of Monte Carlo methods to nuclear medicine problems can therefore be seen as advantageous. 123I is a commonly utilised radionuclide for nuclear medicine diagnostic imaging and non-imaging investigations. The radioactive decay scheme of 123I presents particular challenges in terms of accurately determining the activity of an 123I based radiopharmaceutical prior to administration. The aims of this project were to use Monte Carlo modelling techniques to investigate the uncertainties associated with 123I assay and thyroid uptake assessment using this radionuclide. A model of the UK secondary standard calibrator was created using MCNP5 code and validated against the physical calibrator. A sensitivity curve for the instrument was created through simulation and the effect of measuring 123I using a vial and a syringe investigated. Up to 21% variation was seen between the vial and the syringe geometries studied. A thyroid uptake counter model was produced to determine uncertainties in thyroid uptake assessment for a number of different parameters. Variations in collimator to Page | iv neck distance, horizontal displacement of the detector and increasing depth of the thyroid gland in tissue were shown to affect the accuracy of uptake measurement. A depth-based correction for the thyroid gland was derived from the differential counts in the x-ray and gamma peaks of 123I. Such an approach could be utilised in clinical practice to correct for the depth of the thyroid gland in the neck.