Thallium is a relatively toxic element. The absorption of soluble thallium salts is rapid and complete, which probably contributes to its high toxicity. Furthermore, thallium apparently accumulates in the human body with age. Accumulation of thallium can lead to autonomic dysfunction, with tachycardia and hypertension. On the other hand, thallium apparently has the ability to serve as the required monovalent cation in the activation of certain enzyme-catalyzed reactions, including pyruvate kinase.
Thallium apparently can be present in high concentrations in plants. Zyka found such plants in a region in Yugoslavia and suggested that thallium was responsible for the toxic affect of the plants on cattle.