The growth of a fine undercoat (cashmere) in goats is a seasonal phenomenon; growth generally begins during the autumn, continues into the winter and is followed by a moult in the spring. The endocrine mechanisms involved in the control of these processes are unclear but the thyroid hormones have been implicated in the control of fibre growth (Ferguson, Schenckel, Carter and Clarke, 1956; Lincoln, Klandorf and Anderson, 1980) and in the normal seasonal cycle of reproductive activity in sheep (Follett and Potts, 1990). However, it is not known whether or not the thyroid hormones have a role in the mediation of photoperiodic effects on secondary hair follicle activity in goats.

Most of the metabolic activity of the thyroid hormones is thought to be attributable to triiodothyronine (T3) which is primarily derived from thyroxine (T4) by a process of monodeiodination. This process can be inhibited by treatment with methylthiouracil.

The aim of this study was to suppress the synthesis of T3 and so to determine whether or not it has a direct role in the control of seasonal variation of secondary hair follicle activity and cashmere growth and moulting.