The parathyroid glands are mainly composed of a large number of principal cells, a small number of eosinophils and a stroma. The principal cells secrete parathyroid hormone. Eosinophils may be aging principal cells and are not secretory under normal conditions. The principal cells are lined with fatty granules and the stroma contains adipocytes. The specific gravity of hyperfunctioning parathyroid glands (hyperplasia or germinal adenoma) is higher than that of normal glands because they disappear in the main cells. The specific gravity method can identify normal and hyperfunctioning parathyroid glands. Zhao Lin, Department of Endocrinology, PLA 309th Hospital The parathyroid glands secrete thyroxine (PTH). The parathyroid glands have the following effects: 1. Promote the reabsorption of calcium by the proximal renal tubule, resulting in a decrease in urinary calcium and an increase in blood calcium. 2. Inhibit phosphorus absorption by the proximal tubule, resulting in increased urinary phosphorus and decreased blood phosphorus. 3. Promote the decalcification of osteoclasts to release calcium phosphate Ca3PO4 from the bone matrix and increase the concentration of blood calcium and blood phosphorus. 4. Promote the hydroxylation of vitamin D to produce active 1,25 dihydroxy D3, which promotes the absorption of calcium from food in the intestine. The synthesis and release of parathyroid hormone is controlled by the serum calcium ion concentration, and there is a negative feedback relationship between the two. Low blood calcium stimulates the synthesis and release of parathyroid hormone, which increases blood calcium, and high blood calcium inhibits the synthesis and release of parathyroid hormone, which shifts blood calcium to bone and lowers blood calcium. These effects maintain blood calcium in normal individuals within the normal range. In normal individuals, there is an inverse relationship between blood calcium and blood phosphorus, with high blood calcium resulting in low blood phosphorus, and the product of blood calcium and blood phosphorus is balanced and maintained at 35-40.