It has physical half-life of 192 hours (8 days). However, the effective half-life is a function of the biological half-life of every I-131 compound. I-131Na (sodium compound) is used as treatment or for thyroid testing. It has an effective half-life of 11 hours. It accumulates in thyroid tissue and breast milk. If the dose is higher than 1 mCi breastfeeding should be interrupted. Even more, it would be desirable delaying the procedure for 40 days after nursing has stopped since the mammal gland may result affected. If the dose used was lower than 1 mCi (37 MBq) breastfeeding may be resumed after 41 days of interruption. I-131 OIH (sodium ortho-iodide hyppurate) is used for testing. It has an effective half-life of 6 hours. By a nursing interruption of 30 hours there is no risk of irradiation in the infant. For lactating mothers it is recommended I-123 as a compound for thyroid testing. It would be better the use of a Tecnetium containing compound which is a substance with shorter half-life. Whenever the continuation of breastfeeding is desired, it is advisable to start pumping and milk may be used to feed the infant while nursing is being interrupted whether alone or together with formula. Once the test is over, mother’s milk may be pumped and storage in a separated freezer until it achieves 10 physical half-lives at which point the radiation has disappeared and milk become safe. Radiation level in breast milk can be tested. The shortest half-life radio-nuclide must be chosen whenever possible.