The effect of thyroxine on craniofacial morphology in the growing rat. Part I: A longitudinal cephalometric analysis.

The wide therapeutic use of hormones enhances the importance of detailed knowledge of the effects on skull growth. Thyroxine is known to regulate maturation of cell populations and long bone growth, but the influence on craniofacial bone remodelling is still obscure. One side effect of substitution therapy with thyroxine in children is premature closure of cranial sutures. The aim of the present study was to describe longitudinally the effect of different serum levels of thyroxine on craniofacial growth. In young rats injected with 20 micrograms/kg b.w. l-thyroxine for 28 days a change was found in craniofacial morphology measured by a cephalometric technique. A decrease in length of the viscerocranium was evident after 14 days, and occurred in connection with raised serum levels of thyroxine. In contrast, cranial base growth was increased between day 14 and 28 and also a catch-up growth in the viscerocranium was observed during this period. This observed change occurred after the induced peak in levels of thyroxine in serum had subsided. The inhibited forward directed growth of the viscerocranium was consistent with the fact that the skull in the thyroxine group developed a klinorhycial form in contrast to the normal orthocranialization. Thus, the craniofacial growth pattern was influenced by variations in the serum level of thyroxine. The effects of thyroxine on skull growth sites observed are consistent with reported effects of low dose of thyroxine on long bone growth.