1.5 is the edge of toxicity for Lithium. Its not hard to find the information either.
Even wikipedia - http://en.wikipedia.org/wiki/Lithium_salt
"Lithium salts have a narrow therapeutic/toxic ratio and should therefore not be prescribed unless facilities for monitoring plasma concentrations are available. Patients should be carefully selected. Doses are adjusted to achieve plasma concentrations of 0.6 to 1.2 mmol Li+/litre (lower end of the range for maintenance therapy and elderly patients, higher end for pediatric patients) on samples taken 12 hours after the preceding dose. Overdosage, usually with plasma concentrations over 1.5 mmol Li+/litre, may be fatal and toxic effects include tremor, ataxia, dysarthria, nystagmus, renal impairment, and convulsions. If these potentially hazardous signs occur, treatment should be stopped, plasma lithium concentrations redetermined, and steps taken to reverse lithium toxicity. The most common side effects end up being an overall dazed feeling and a fine hand tremor. These side effects are generally present during the length of the treatment but can sometimes disappear in certain patients. Other common side effects such as nausea and headache, can be generally remedied by a higher intake of water. Lithium unbalances electrolytes; to counteract this, increased water intake is suggested."
Whoever told you what you just said is dead wrong, and if they are a doctor they should be struck off because trying to get a patient to 1.5mmol is bloody dangerous and stupid and a great way to kidney and liver damage - a 1.3 or 1.4 reading would be enough to panic just about any pdoc in the land as its far too close to toxic - 1.5 would panic just about any doctor.
The theraputic range is 0.6 to 1.2 and commonly 0.8-0.9 are where a pdoc will want a patient - 1.2 is very high in adults as its easy to become toxic and thus a pdoc would normally only want a patient at 1.2 if they were paedeatric or severely manic.
Yes everyone is different, and for some patients 1500mg is low - I take1250mg myself and I have a friend who takes 1750 and we sit t 0.8 to 0.85 normally - lithium is a salt and its very dependant on hydration and other things - James only has 1 kidney and this is going to affect his results as well.
I know this is hard but seriously if youre going to give people advice please check it on other sources.. The theraputic range and information on lithium salt is the single easiest thing to find on the web and its not a new drug - giving possibly dangerous advice is a bad idea to someone seeking information.
Lithium is one of those meds that you just have to play with. It's a trial and error type of med. Honestly, even a level of 0.9 is not therapeutic for bi-polar disorder. It should be at least 1.0 if not up to 1.5. And 0.6 is definitely not therapeutic. I was on 1500mg of Lithium once daily, extended release, and I still wasn't at a therapeutic range. Everyone is different. My metabolism just happens to metabolize lithium at a higher rate than it does for others. That might be your issue.