The effects of thyroid hormones on myelination in the developing rat brain.

Rats radiothyroidectomized 1 day after birth received daily subcutaneous injections of 1 microgram/10 g body weight of thyroxine (T4) or an equimolar amount of triiodothyroacetic acid (T3AC) from day 6 through day 25. The number of myelinated axons, myelinated axon area, and area of the myelin sheath in the corpus striatum were investigated. Hypothyroid neonates demonstrated a normal number of myelinated axons with a decrease in the area of these axons. T4 treatment resulted in an increased number of smaller axons while T3AC treatment produced fewer but larger axons than the T4 treatment. The myelin area changed as the axon area changed with the myelin thickness remaining constant in all groups.