From my reading, one would take a large dose of iodine and then see what is excreted in urine. If most was not excreted, your iodine stores were low and uptake was high. If most is excreted, iodine levels can be considered close to saturated. I am really not deeply experience with this and I believe that some simple tests [yours] do not tell the whole story. In any case, my recent experience where taking a lot of iodine resolved a lot of problems was also inexpensive, and diagnostic.
Now your iodine test has a range of 28-544. That is mean plus/minus two standard deviations or something like that. Given that around 90% of the USA population is iodine deficient, what does this tell us or tell us about your result? You are at the low side of what is seen in a deficient population. There is still nothing there that tells you what is really healthy or optimal.
Lab "normal" ranges are artifacts of a test population and do not have anything to do with healthy or normal. Almost all doctors have that wrong or are otherwise stupid. A few lab ranges were changed to reflect medical recommendations, PSA, homocysteine, glucose, cholesterol... While that is valuable, it starts to make more think that all lab ranges represent a normal state of heath.
I don't see the point of chasing iodine lab ranges. Save your money and get some improved iodine intake. My wife found a dried seaweed product that you sprinkle like salt, it is salty, contains 3mg of iodine per 1/4th teaspoon, 20 times your RDA. Maine Coast Sea Vegetables Inc www.seaveg.com