Stages in goitre formation are:

● Persistent growth stimulation causes diffuse hyperplasia; all lobules are composed of active follicles and iodine uptake is uniform. This is a diffuse hyperplastic goitre, which may persist for a long time but is reversible if stim- ulation ceases.

● Later, as a result of fluctuating stimulation, a mixed pat- tern develops with areas of active lobules and areas of inactive lobules.

● Active lobules become more vascular and hyperplastic until haemorrhage occurs, causing central necrosis and leaving only a surrounding rind of active follicles.

● Necrotic lobules coalesce to form nodules filled either with iodine-free colloid or a mass of new but inactive follicles.

● Continual repetition of this process results in a nodular goitre. Most nodules are inactive, and active follicles are present only in the internodular tissue.