Q's re. Kidney and Thyroid Function Test Results
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Q:  Q's re. Kidney and Thyroid Function Test Results

Hello Dr. Phelps!

Thank you for the informational website and all of the help you provide.

I have stopped and started Lithium a couple of times over the last 2.5 years.  I currently am on Lithobid 1200mg.  A recent blood test indicated a Bun of 13 (7-25 mg/dl) and a Creatinine of .9 (.5-1.2 mg/dl).  It also indicted that my Estimated GFR was >60 and my TSH 3rd Gen as 3.43.

I put the values into one of the online GFR calculators and it indicated a GFR of 73.  According to what I read this would indicate CKD stage 2.  Naturally, I am panicking.

Am I making too much out of this (jumping to a conclusion) or should I start looking for alternative therapies?

Also, why do they report the GFR as >60 instead of giving a precise number?

Thank you very much for any information that you can provide!

Dear Cheri --

First of all, you are taking quite appropriate steps to monitor your kidney function and thyroid function. These bear watching in anyone taking lithium.

And, secondly, yes, you are making too much out of this and you do not need to start looking for alternative therapies, yet.

For other readers not as up on this as you are, here is a quick review on calculations of kidney function, and an attempt to address your question about how GFR is reported. 

Creatinine is a breakdown product of muscle which is slowly and constantly released into the bloodstream as muscles reshape themselves. The kidneys clear it from the bloodstream. When the kidneys slow down, creatinine goes up. Therefore we can use creatinine, which is obtained in a simple and very standard blood test, as a marker of kidney function. 

GFR refers to "glomerular filtration rate", which  means the actual rate at which the blood is being filtered by the kidneys. in other words, this is what we really want to know: how well are the kidneys working?  How efficiently are they doing their job?  Can they handle a lot of filtering, or are they slowing down to a lower filtration rate? 

GFR can be measured indirectly in several ways.  In the old days, we used to have people gather all the urine they produced in 24 hours and bring it to the lab for analysis. As you can imagine, this was a hassle for everyone involved.  More recently, the laboratory computers take your age, race, gender, and sometimes body weight into consideration. This is a fancy version of a formula that has been around for a long time to calculate GFR (the Cockroft-Gault formula). I do not know the answer to your question regarding why GFR is reported as greater than 60 by the laboratory computer, instead of a more precise number.  I suspect that beyond 60, the calculation is too inaccurate to be precise, so rather than mislead people with a specific number, it is reported this way.

Now, back to the main issue...

Lithium, used long-term, is known to decrease kidney function: the GFR goes down, and creatinine goes up. This happens to about 10% of people who take lithium for a decade or more.  It does not happen to everyone, and it definitely does not happen quickly: it takes years and the problem develops slowly (the exception is a rare immediate reaction to lithium, which is a different phenomenon).  So there is plenty of time, at your stage, to decide on what to use after lithium.  The question is, when should you "pull the plug" on lithium treatment?

In a recent review issue devoted entirely to kidney issues and psychiatry, I saw a specific article examining just how high we should let creatinine go before we think about tapering off lithium. To my surprise, even in this specialist review, the number was much higher than I expected. indeed, it was the same number I found one I went looking for this a year or so ago: 1.6 mg/dl (on which scale you are currently at 0.9).

Now for my opinion. In people whose creatinine has clearly been going up over the years, say from 0.7 to 1.0 or 1.1, I think we can fairly confidently conclude that if we leave them on lithium, that trend will continue. In that case, ultimately we are going to be taking them off lithium. So, why don't we go ahead and do that when their kidney function is still quite good, rather than waiting for a creatinine of 1.6, which is well outside the normal range and marks a significant deterioration in GFR --?

In other words, why wait for 1.6? Why not move on to an alternative at 1.1 or 1.2?

I have this discussion with my patients when they reach 1.1 or so. Unfortunately, for many of them, lithium has been literally  life-saving. It is frightening to move on to another treatment which may or may not work as well, especially when the consequences of a treatment that does not work can be really quite disastrous (several of my patients who have faced this dilemma have a history of very dramatic and dangerous manic episodes, for example). How do you know that the next treatment will work at all? You really cannot notice (unless you had previously had a good response to something else; but in that case, why are you on lithium? Something about that other treatment was not good enough).

The bottom line: there is no specific creatinine level at which you should taper off lithium. Instead, when your creatinine gets a little higher, you can take your time about it, but have a discussion with your treating physician about what to move on to. Why not do that now? First of all, some of my patients have had a creatinine of 0.9 which has subsequently gone back down to 0.7. The test just wobbles around quite a bit. So first of all, you would want to have established that there was a clear upward trend in creatinine. Otherwise it is quite safe, based on everything we know about lithium, to continue taking this medication when you have a creatinine of 0.9. 

In my experience, it can take several years to establish this upward trend. During that time, in theory you can relax about this issue. In practice, some people will find themselves still constantly worrying about it. Factor that into your discussion with your psychiatrist. It depends, in a way, on how well lithium has worked for you. If it has been fantastic, and there are few alternatives (for one reason or another), it might be best to try to come to terms with your anxiety, rather than switching. You get the idea. Good luck with figuring out how to proceed from here.

Dr. Phelps

Published April, 2008


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