Lithium- Blood Tests & Interpreting TSH Levels
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Q:  Lithium- Blood Tests & Interpreting TSH Results

I have a question about when is it necessary to check lithium blood levels?  I am taking 600mg of Lithium (w/ no side effects) and my psychiatrist insists that that's way below the toxic blood levels and hasn't tested my blood.  At what point should it be reccommended that my blood levels are tested?  I think my doctor may increase my lithium since the plan is to wean me off celexa and possibly increase the lithium.  He started at such low doses of lithium because I am not clearly bipolar--but I definitely do not fall into the standard depression or anxiety diagnoses.

I hope to hear from you soon.  Please reply to my email, reguardless of whether my question goes on your site.

And I love your website (psycheducation)  I have learned sooooo much!


Dear LM -- 
Well, you're both right, basically.  I often delay getting a blood test if I'm not going above 600 mg of lithium.  

And, it would be good to do a blood test now and then; but the main things to look at would not be lithium level (although I'd get one as long as you were getting your blood drawn) but thyroid (TSH) and a "baseline" measure of your kidney function.  Then, if you were doing well and the medication was to be continued, I'd look at your TSH again at least once a year, at least for the first year or two.  I'm starting to check that TSH even more often as I get more interested in how thyroid affects bipolar disorder, in order to catch very early those folks who are going to head toward hypothyroidism on lithium.  I don't wait for them to get formally hypothyroid anymore, but instead start thyroid replacement of one kind or another as soon as it's clear that they're drifting north, i.e. that their TSH is going up.  

In case you need it for those purposes, here's how to interpret TSH: 

Interpreting TSH results

TSH is the signal from your pituitary gland, up there in the middle of your brain just under your hypothalamus,  that tells your thyroid gland to "MAKE MORE" thyroid hormones.  When your gland makes too little hormone, TSH goes up.   It's yelling:   "I said, make more thyroid hormone!"  So a high TSH goes with hypothyroidism, meaning too little thyroid hormone. 

And, you guessed it, a low TSH happens when your brain is seeing a lot of thyroid hormone.  It no longer has to yell, it's whispering now: "ok, ok, that's plenty already".  And if your thyroid levels were really high, the TSH would go down so low we might not even be able to find it with a lab test. 

Ok, well how low a TSH can you get and still be okay?  (Remember, as discussed above, and in more detail here, becoming hyperthyroid means increasing your risk of osteoporosis).   That is, how far can you push your thyroid dose up, before the TSH would go so low you would be "hyperthyroid" and therefore taking risk with your bones?  Answer: nobody knows for sure.  One research group has shown people who were maintained "hyperthyroid" didn't get bone density decreases we usually would have expected.Gyulai

For now, to be cautious, at least we shouldn't let your TSH go below the lower limit of "normal" (0.4 uIU/mL -- forget the units.  Some labs now have a lower limit of 0.2, because their test is different, so check the reference range on your test).   Below that, most experts would be worried about your bones.  And there are some who would not let your TSH get below the middle of the normal range due to this risk (around 1-2 is the "bottom" for many doctors I've worked with).

Dr. Phelps

Published October, 2002


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