Thyroid hormone secretion rate and milk yield in lactating rats

¹ Department of Physiology, University of Tennessee, Memphis, Tennessee

Thyroid secretion rate (TSR) obtained from primiparous lactating rats during the first 14 days of lactation averaged 1.93 µg/100 gm/day l-thyroxine or 130% more than obtained from nonlactating female rats of the same age and strain. TSR was significantly correlated with the amount of milk obtained from mothers on day 14 of lactation. TSR obtained from the same rats during a second 14-day period of lactation (nursing a foster litter) was significantly reduced to an average of 0.98 µg/100 gm/day l-thyroxine. A significant reduction in milk yield on day 28 accompanied the reduction in TSR. The reduction in milk secretion during the second 14 days of lactation resulted in slower growth of the foster litters. Maternal body weight was unaltered. In a second experiment the average TSR of 2.10 µg/100 gm/day obtained during the first 14 days of lactation fell to 0.75 µg/100 gm/day during a subsequent 14-day period of nonnursing. The intensity of lactation apparently is associated with the intensity of thyroid hormone output, and inadequate thyroid hormone secretion may be an important factor in the decline of milk secretion.