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Guessing vs. calculating
Here are some examples to illustrate how it is impossible to truly know how much I-131 a specific patient will need without actually fully evaluating them and calculating the dose based on the T4, T3, Tc-99 thyroid uptake, tumor volume, and equally important based on the 24h I-131 uptake. You will see below that no one parameter will provide a reliable correlation to the dose needed to treat.
Individualized dosing is not truly individualized unless you consider all of the parameters above. Unfortunately, calculating the patient's individual I-131 dose requires expertise and advanced equipment. This is a labor intensive and time consuming process making treatment more expensive than when performing a fixed dose approach that only requires an injection and boarding until patient can go home (according to radioactivity level).
2 cats treated the same day with very different expected I-131 doses
The cat on the left column was expected to need a much lower dose than the cat on the right column. The cat on the left had a lower T4, lower T3, lower tumor volume, lower Tc99 thyroid uptake, and a detectable TSH. However, this cat ended up needing a higher dose because of its very low 24 hour I-131 uptake. In essence, this cat's thyroid gland excreted the I-131 much faster than typical cats. Therefore, the I-131 needed to be adjusted and this cat ended up needing a higher dose than the other cat.
Very low I-131 dose needed
In this example, this cat required a very low dose of I-131 in order to correct the hyperthyroidism. Chronic kidney disease was also unmasked in this case. Therefore, a higher I-131 dose would have not only made this cat hypothyroid, but also worsened its kidney disease.