Hyperthyroidism in animals has been associated with early onset of secretory activation (lactogenesis II), and lower levels of oxytocin and prolactin at milk ejection, lower volume of milk released at milk ejection, lower weight gain in the pups and milk stasis leading to inappropriate/early involution (Varas et al 2002). Some women report abundant milk production while hyperthyroid, but others seem to have no functional milk ejection reflex and consequently lactation fails (West & Marasco 2009). The reason for the difference is not yet understood.