My 10 year old daughter (type 1 diabetes for three years) has had puzzling blood work results recently. Three months ago, her TSH reading was 9.3 and now it is 3.9 (thereby avoiding for now the use of thyroid hormone), but three months ago, her anti-TPO antibodies were 747 and now they are 498, and her anti-microsomal antibody result was 131 two months ago. Should this test result be negative? Can you explain the significance of high anti-TPO readings? Should her tests be repeated regularly? How often?

I can understand how you might have been confused by these laboratory results. To begin with, in interpreting any value you need to know both what the normal range is for that particular laboratory as well as what units they are using. On our scale, the first TSH value would be distinctly abnormal though perhaps not quite high enough to justify starting thyroid hormone replacement therapy immediately especially if the gland was just diffusely enlarged, whereas the 3.9 result later is comfortably within normal limits.

As to the antibody tests, TPO or Thyroid Peroxidase Antibodies are synonymous with Microsomal Antibodies; our normal range is <0.3 units/ml (<300 mU/ml or units/liter with re-testing suggested in the 300 to 1000 mu/ml range). I suspect that the laboratory that your daughter's sample went to expressed the result in mU/ml or units/liter which would which would put her anti-TPO figures in the indeterminate range; but not yet in the abnormal range. All of this suggests that your daughter has some rather equivocal and perhaps transient evidence of early hypothyroidism which does not need treatment now and may never; but that the TSH should be repeated every three months or so until the matter is decided. Personally I do not think that the antibody tests need to be done again because the chances of any loss in thyroid function having other than an autoimmune cause are very small. DOB