Sodium

Sodium is the salt your body keeps outside the cells, in your blood and the fluid around it. It attracts water, and with that it determines how much water sits in your blood vessels and how much in your cells. Your brain guards that ratio with two levers: thirst, and a hormone that instructs the kidneys to hold on to water or release it. If you eat salt, you get thirsty and your kidneys hold on to water until the ratio is right; if you drink too much, you pass the surplus within hours. The value in your blood therefore hardly changes with what you eat. What does shift it is a body that cannot get rid of water or cannot hold on to it. That is why a low sodium is about too much water and not about too little salt. Diuretics, heart failure, a liver or kidney disease and a hormone working too hard make the kidneys hold on to water, and the sodium dilutes. A high sodium is about too little water: fever, diarrhoea, heat and not drinking enough, and then thirst is the first signal. Because your brain cells swell as sodium falls and shrink as it rises, a fast shift gives headache, nausea, confusion and in the extreme a seizure. A slow shift you tolerate much better, and a slow correction is therefore safer than a fast one. Your report shows sodium in mmol/L. Normal is about 136 to 145 mmol/L, where the labs overlap, a band of less than ten units. Below 130 mmol/L and above 150 mmol/L the requester is phoned, because the brain then gets into trouble.

Sodium is the most tightly regulated value in your blood, and exactly for that reason a deviation says a lot. Anyone who takes diuretics, is older, or has a heart, liver or kidney that works less well can sink to the low end unnoticed, and you only notice when you become tired, nauseous or confused. In older people a low sodium is one of the most common explanations for falls and confusion, and it is an explanation you find with a blood value and solve with an adjustment of the medicines or the drinking. For anyone who does sport, sodium is the value of the long effort. Anyone who drinks only water for hours during a marathon, a long cycle ride or a hike in the heat dilutes the sodium while sweat carries salt away, and a low sodium is more dangerous in endurance athletes than dehydration: it gives nausea, headache and confusion that look like exhaustion, and with a fast fall the brain swells. Drinking to thirst, and with an effort longer than a few hours salt or a sports drink alongside, is what keeps the value up. On the other side, a high sodium is almost always dehydration, and in anyone who can drink unaided thirst solves that. The problem sits with anyone who cannot: a baby, someone ill with fever and diarrhoea, or an older person with a disturbed sense of thirst. There a high sodium is a sign that too little water is coming in, and the solution is drinking, not less salt.

A low sodium means too much water relative to the salt, and the question is why the body holds on to the water. In order of how often it occurs: diuretics, especially the thiazide kind; heart failure, a liver disease or a kidney disease, in which the body holds on to water while the blood becomes diluted; and diarrhoea or vomiting in which you drink back only water. After that come a markedly raised blood sugar, which draws water out of the cells into the blood; medicines such as antidepressants, antipsychotics and drugs for epilepsy; litres of water during a long effort; and a hormone that makes the kidneys hold on to too much water, with lung diseases, brain conditions or some tumours. How fast the sodium fell counts more than how low it is: a value that dropped below 130 mmol/L within a day is an emergency, the same value that has been there for months usually is not. A high sodium means too little water. Usually it is dehydration from fever, diarrhoea, heat or drinking too little, and in anyone who can drink unaided it recovers with water. In anyone who cannot, it is a sign to help with drinking or to give fluids. Rare is a kidney that cannot hold on to water because the hormone is missing or does not work, and you recognise that by litres of clear urine with a thirst that does not stop. Always read sodium beside potassium, because diuretics and adrenal hormones shift the two together, beside creatinine and the eGFR, which say whether the kidneys can release water, beside glucose, which lowers sodium when it is high, and beside albumin and triglycerides, which at extreme values can distort the measurement itself.