Cesium.

Cesium is a potassium congener carried by the same sodium-potassium ATPase and potassium channels, so it displaces potassium at those sites and lowers blood potassium. Anyone taking cesium is altering potassium handling, which makes this an interaction to respect rather than a benefit.

Rubidium is the other alkali metal that substitutes for potassium on the same pump and channels. Taking both adds competition at one site rather than covering two different ones.

Magnesium-dependent pumps and channels decide how well a cell holds potassium and keeps a normal membrane potential. Magnesium status is therefore part of the picture whenever cesium is displacing potassium.

Caesium enters cells through pathways built for potassium and sodium, since the ion carries the same single positive charge and differs mainly in radius. Because it is poorly extruded by the sodium-potassium ATPase compared with the ion it displaces, the balance of monovalent cations across the membrane shifts. That relationship is a competition, not a helpful pairing.

Layered aluminosilicates including bentonite, illite, vermiculite and mica hold caesium tightly at interlayer and frayed-edge sites, which is why these minerals dominate how caesium moves through soil. In the gut, a clay would be expected to bind luminal caesium and reduce what is taken up. This is ion-exchange chemistry and it also means a clay binds other cations you may want, including potassium and iron.

Caesium exposure disturbs the monovalent cation balance that sets resting membrane potential, and calcium currents govern the plateau and contraction phase of the cardiac action potential alongside it. The two therefore interact at the level of cardiac electrical stability rather than in the gut. Any electrolyte disturbance in that setting is a matter for a clinician, not a formulation decision.

Stable iodine saturates thyroid uptake and is the recognised countermeasure for radioiodine exposure. It does nothing for caesium, which is not concentrated by the thyroid and instead distributes with the body's potassium pool through muscle. The pairing is listed to correct a common conflation, not because iodine acts on caesium.

Selenium supports glutathione peroxidase activity and is often discussed generically alongside radionuclide exposure. No caesium-specific interaction is established in the material available here. The row exists to state that absence plainly rather than to imply an effect.