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Parathyroid hormone is not required for normal milk composition or secretion or lactation‐associated Bone loss in normocalcemic Rats

To determine if parathyroid hormone (PTH) is essential for lactation in rats, the parathyroid glands were removed surgically during the first week of lactation and the rats were given a diet containing a high calciumphosphorus ratio to maintain a normal serum calcium concentration. Lactating rats were placed on diet containing 1.2% calcium (Ca) and 0.8, 0.6, or 0.4% phosphorus (P) on day 2 postpartum (PP) and were parathyroidectomized (PTX) at 4–6 days PP. At 10 days PP serum Ca was 10.5 ± 0.2 mg/dl (mean ± SEM) for PTX rats and 10.4 ± 0.3 mg/dl in sham‐operated lactating rats when the diet contained 0.6% P. When the diet P was 0.8%, the litters gained little or no weight and serum Ca fell to 6.9 ± 0.6 mg/dl by day 10 PP in PTX rats compared with 10.2 ± 0.2 mg/dl in sham rats. PTX rats fed the diet containing 1.2% Ca and 0.6% P maintained a normal serum Ca level until at least day 18 PP, but their serum P levels fell gradually from approximately 5 mg/dl at 10 days to 3 mg/dl at 18 days PP. In spite of this hypophosphatemia, the litters of PTX and sham rats had gained the same amount of weight by age 16 days, indicating equal milk production in the two groups. Milk Ca, P, and total solids were not significantly different between PTX and sham rats on day 11 PP. With the 1.2% Ca and 0.4% P diet, the femurs lost approximately 20% of dry bone mass in both PTX and sham rats (p < 0.01) between 5 days PP, when PTX was carried out, and 20 days PP. We conclude that the absence of the parathyroid glands does not affect milk production, milk mineral composition, or the lactation‐associated bone loss when normal serum Ca levels are maintained by diet. We suggest that PTH is not essential for lactation beyond its normal function of maintaining the serum Ca concentration