Is there anything that is completely non-toxic to humans at any dose?
I’ll answer this theoretically, since that’s how it has been posed. And if we’re ignoring practicalities, we may as well posit that the substance in question will be introduced directly into the bloodstream (This is, of course, simple to do in reality, but not how most people consume their non-toxic substances.) The easiest way to show that any unspecified substance can be toxic at an unlimited volume is to invoke the human body’s mechanisms for volume homeostasis.

As mentioned in this answer, the human kidneys functioning optimally can produce up to ~ 25 L/day of urine.1 This would require complete suppression of ADH (anti-diuretic hormone, a.k.a. arginine vasopressin), which would occur only if the “toxin” load were markedly hypotonic (think water).2  There is therefore a theoretical maximum volume of any substance that can be dealt with by the body, which is something less than 25 L per day. (For any substance other than water, the maximum will be lower because ADH will not be as fully suppressed by a less hypotonic load.) 

A volume of any substance introduced into the bloodstream (including a product precisely mimicking the constituents of the bloodstream itself!) will overwhelm the body’s homeostatic mechanism. This will result in edema which is unpleasant and, in the case of pulmonary edema, certainly pathologic - a “toxidrome” in your scenario. In the case of hypotonic fluids, serum osmolality will also fall causing hyponatremia with all of its consequences.

Summary: No, the human body can not tolerate an unlimited volume of anything, therefore there is no substance that is non-toxic "at any dose." 




1. Christopher Lote. (2012). Principles of Renal Physiology. Springer New York.
  


2. No, you may not drink 25 liters of water per day. For one thing, urine can not be made with a tonicity of 0 to balance this (more like 60 mOsm/kg minimum). Additionally, ADH can rarely be completely suppressed, yielding a somewhat more concentrated urine and therefore lower tolerance for hypotonic intake before serum osmolality is compromised.