When the TSH test reveals abnormal results, a doctor may order the T4 test. It measures the quantity of thyroxine inside blood. There are two types of T4 tests. One that measures the free thyroxine and something that measures the complete thyroxine available. The total thyroxine test is impacted by proteins which can be guaranteed to thyroxine. The free thyroxine test is known as more accurate since it isn't afflicted with protein levels. Identical twins originate from one particular egg that's been fertilized by one sperm. For unknown reasons, the fertilized egg splits into 2 or more embryos during the first stage of development. Some identical twins share the same placenta. However, they generally grow within separate amniotic sacs inside the uterus. In rare cases, identical twins share one amniotic sac (monoamniotic).