Use of (3H) and (125I) tracers in steroid radioimmunoassays

The comparative use of 3H and 125I tracers in steroid radioimmunoassays will be discussed around the following points: 1. 3H. a)

they can be purchased commercially and have a long shelf-life. b) Disadvantages: they may have reduced affinity for antibodies due to "isotope effects"; the counting of beta-emitters is more expensive and difficult; 3H tracers are not available for all steroids. 2. 125I. a)

gamma-counting is cheaper, simpler and more precise; 125I tracers may have higher affinity for antibodies than unlabelled steroids; 125I can be used to label any steroid. b) Disadvantages: 125I tracers have a limited shelf-life (n.b. six months for 125I histamine tracers). The high affinity of some tracers will be a big disadvantage if the unlabelled steroid cannot compete effectively.