Hormones and Calcium Metabolism 237 



1. The thyroid hormone raises plasma calcium, depresses parathyroid activity and 

 increases tubular reabsorption of phosphate. Urinary calcium is increased both be- 

 cause of an increase in filtered load and reduced parathyroid activity. Calcium ab- 

 sorption is reduced (? due to reduced parathyroid activity). Hypercalciuria and 

 malabsorption of calcium lead to negative calcium balance and osteoporosis or osteitis 

 fibrosa according to severity. 



2. The corticosteroids depress plasma calcium, reduce calcium absorption, increase 

 calcium excretion and depress plasma and urinary citrate. If citrate were an intra- 

 cellular calcium carrier it might be suggested that these steroids reduce calcium 

 transport. Malabsorption and hypercalciuria would suffice to explain the osteoporosis 

 but a primary efFect on bone protein cannot be excluded. 



3. The oestrogens depress plasma calcium, reduce urinary calcium, raise urinary 

 citrate, probably increase calcium absorption. They may promote calcium transport, 

 possibly through the citrate ion. They tend to stimulate the parathyroids and so cause 

 phosphaturia and hypophosphataemia and this contributes to (or may explain) their 

 hypocalciuric action. Loss of oestrogens allows plasma and urinary calcium to rise 

 and so reduces parathyroid activity and increases urinary calcium. Calcium absorp- 

 tion may fall. These effects result in negative calcium balance and osteoporosis but a 

 primary effect on bone resorption cannot be excluded. 



4. Growth hormone raises plasma calcium, reduces parathyroid activity, raises 

 plasma phosphate, increases urinary calcium. The effects on plasma and urinary 

 citrate and on calcium absorption are uncertain, but hypercalciuria causes negative 

 calcium balance and osteoporosis despite the stimulating effect of the hormone on 

 new bone formation. 



When these actions (which are summarised in Fig. 7) are considered in relation to 

 those of the parathyroid hormone (hypercalcaemic) and calcitonin (hypocalcaemic) 



Growth h k S \ ? N I 



Fig. 7. Tentative scheme indicating possible actions of hormones on various aspects of calcium metabolism 



a pattern emerges in which the plasma calcium is the resultant of the opposing effects 

 of at least six hormones on bone, 3 tending to raise it and three to lower it. However, 

 although all these six hormones (and possibly others) may influence plasma calcium, 

 only the parathyroid hormone (and possibly calcitonin) is in all probability secreted 

 in direct response to variations in plasma calcium concentration. The intervention of 

 these other hormones presumably makes the work of the parathyroid glands even 

 more critical than it would otherwise be. 



