What is the mechanism of action of lithium-induced polyuria?
Lithium changes the effect of at least three substances that play a role in diuresis:


aldosterone:  lithium partially inhibits its ability to increase the expression of ENaC receptors on apical membrane thus increasing sodium losses [1].
arginine vasopressin (AVP): because lithium can induce hyperparathyroidism, parathyroid hormone can act as partial agonist to AVP, inhibiting its hydroosmotic action [2].
antidiuretic hormone (ADH): lithium blocks its stimulatory effect on adenilate cyclase. The result is a decrease of cAMP levels in the cells from collecting tubules. cAMP promotes formation of water channels, thus improving absorption from collecting tubules. Because lithium decreases cAMP levels, diuresis is stimulated [1].


All these result in polyuria.

Lithium levels can be determined from the blood. Usually the test should be performed 12 hours after last dose of Lithium [3].



References:


Eleanor Lederer, MD; Lithium Nephropathy on http://emedicine.medscape.com/article/242772-overview#a0104
Carney SL, Ray C, Gillies AH. Mechanism of lithium-induced polyuria in the rat. Kidney Int. 1996 Aug;50(2):377-83.
http://labtestsonline.org/understanding/analytes/lithium/tab/sample/