The endocrine system is complicated and dynamic. Thyroid hormones (T3 and T4) aren't anabolic in higher doses (they will actually cause muscle loss in high doses), but in people with extremely low levels of thyroid hormones, there is growth stunting. I;m not sure why this happens, possibly due to various hormone interactions. So, in moderation, they may be anabolic.
Here, it talks about how hypothyroidism can slow bone growth, while hyperthyroidism can do the same, by causing the body to burn off too many sources of energy.
I just found this by doing a quick search of thyroid and growth in Google. If you go to PubMed, you can find tons of more detailed stuff.
http://healthresources.caremark.com/topic/growththyroid