123-I is a cyclotron product. Cyclotron is a type of particle accelerator that is used to produce 123-I and many other radiotracers.  Commercial cyclotrons are run by large and small radiopharmaceutical companies.  Considering the relatively short half-life of 13 h for 123-I, many geographic locations are not within the reach of a production site.  Additionally, flights delays and other transportation issues as well as custom (Canada is major country of origin for 123-I used in US) may be limiting factors for use of 123-I. 123-I is usually available for oral administration but can be prepared for IV use.  The oral 123-I is mostly supplied only in form of capsules but liquid form for oral and IV administration can be requested.  Of note, since radioiodine vaporizes quickly, in order to limit radiation exposure to nuclear medicine staff, capsule is the preferred formulation unless swallowing of the capsule is an issue.  

Radioiodine is absorbed in the distal stomach and proximal duodenum.  As such, in patients with status post gastric bypass surgery or gastrectomy, IV form should be used.  Oral administration in such cases would artificially decrease calculated percentage uptake in the thyroid and underestimates the radioiodine avidity of local residual tissue and metastatic lesions in thyroid cancer patients. For the same reason, in case of therapy (with 131-I) in patients with status post gastric bypass surgery or gastrectomy, the desired radiation dose will not be delivered.  

123-I is more expensive than 131-I.  As such Medicare and private insurers limit the amount (mCi) they reimburse.  Although this amount may vary from time to time and between different Medicare contractors, it is usually sufficient to produce high-quality images.  The nuclear medicine staff should familiarize themselves with the amount they get reimbursed for, and optimize the imaging protocols (acquisition time, etc) to produce both high quality planar and SPECT/CT images.