Q:  Can the blood level of lithium be affected by strep or flu?  Can weightloss or gain require a dosage adjustment to lithium?

Dear Ms. P.-

The blood level of lithium can indeed be affected by anything which changes your body fluid levels. Having the flu, especially if you are throwing up, but even just diarrhea (worst of all would be both at the same time), can cause you to become very significantly dehydrated. This will change your blood level of lithium substantially. In the next paragraph, I will try to explain why, but the bottom line is: whenever you are having difficulty keeping fluids on board, you should lower your dose or even stop taking lithium until you are hydrating well. Of course, your doctor may want to give you specific directions and if there is any doubt whatsoever, you should call her or him.

Lithium is very similar to sodium, which you know as part of table salt (sodium chloride). Recall that lithium sits right next to sodium in the periodic table of elements. Your body recognizes them as similar as well. Indeed, under circumstances when the your kidneys are trying to hold on to sodium, they will hold onto lithium. The kidney is prompted to retain sodium one you are dehydrated (as water will stick were sodium has gone, so getting more sodium will force your body to hang on to whatever water you have -- as well is making you thirsty!).

Therefore, you can raise your lithium level by becoming dehydrated, even just by sweating and not drinking enough fluids. In general, these lithium shifts are relatively minimal, unless you were to work out hard without drinking anything, for example. But when you have the flu, and are not drinking fluids normally; and may be losing fluids excessively in the form of diarrhea or vomiting; then you are in a position to become quite markedly dehydrated, and then lithium levels can rise substantially.

This is why some doctors recommend to their patients that they not take lithium when they have the flu, at least not until they are taking and keeping fluids normally. If you were to have strep throat, and be taking minimal fluids on that basis (as well as sweating because of fever), that would likely be a similar scenario.

As for weight loss or gain, these are much smaller effects on lithium level. Indeed, the most likely impact on lithium level would be from dehydration during low food intake, if it was accompanied by low fluid intake. On the other hand, people who drink lots of water while trying to suppress their appetite (and perhaps even consuming relatively little sodium) might lower their lithium levels somewhat. These fluid shifts likely would have much more impact than a weight shift.

Thank you for the interesting and important question.

Dr. Phelps