What controls the sodium levels in the body?

Kidney, adrenals, heart, liver, pituitary, can all play a part in salt regulation, as can dietary intake.

the juxtaglomerular apparatus which is a group of cells located between the afferent arteriole and distal convulated tubule which are found in each nephron of the kidney. these cells detects the low sodium level.
when theres low sodium level , they will release the enzyme renin...
from angiotensinogen, it wil be converted to angiotensin I by the enzyme renin.
angiotensin I will be furtherly converted into angiotensin II by a converting enzyme
this action will stimulate the adrenal cortex to release hormone aldosterone which reabsorb more sodium ions from the nephron back into the blood.. this also increases the osmotic pressure which drives water into the blood raising the blood pressure..


hopes this helps.

I would say that blood sodium levels are regulated by the kidneys and the liver.
Also, is the patient taking in as much sodium as they normally do?