A few informative sites about thyroid issues:
"TSH rise in DS may be independent of thyroid disease. There is a transporter problem with TSH and too much of it gets made more than is needed for feedback. There seems to be no effective way to stop this but it should not cause a problem except in worrying people that thyroid disease is there. I have seen more than a few endocrinologists treat the TSH alone and have caused hyperthyroidism in more that 40 patients by doing this."
Reference Ranges Debate
Tests for Thyroid and Related Functions http://www.westonaprice.org/Best-Kept-Secret.html
Age-Related Patterns of Thyroid-Stimulating Hormone Response to Thyrotropin-Releasing Hormone Stimulation in Down Syndrome