IU stands for "International Units" and it is an arbitrarily chosen unit of measure used mostly to quantify hormones, vitamins or various substances found in the blood.
The idea is well explained by the Wikipedia article on international units)
Many biological agents exist in different forms or preparations (e.g. vitamin A in the form of retinol or beta-carotene). The goal of the IU is to be able to compare these, so that different forms or preparations with the same biological effect will contain the same number of IUs. To do so, the WHO Expert Committee on Biological Standardization provides a reference preparation of the agent, arbitrarily sets the number of IUs contained in that preparation, and specifies a biological procedure to compare other preparations of the same agent to the reference preparation. Since the number of IUs contained in a new substance is arbitrarily set, there is no equivalence between IU measurements of different biological agents. For instance, one IU of vitamin E cannot be equated with one IU of vitamin A in any way, including mass or efficacy.
For hCG, in the 55th report for of the WHO Expert Commitee on Biological Standardization, you could find:
The Expert Committee on Biological Standardization, after consideration of this issue (11), concluded that the choice of unit should be made on a case-by-case basis and reflect, and be based on, the biological and medical as well as the physicochemical information available.
Many biologicals exist in both active and inactive states, and the clinically relevant form of the analyte may depend on the diagnostic aim. For example, the active state of the placental hormone chorionic gonadotrophin (hCG) is the relevant molecule to measure in the diagnosis of pregnancy, whereas the biologically inactive free beta subunit (hCG-beta) is measured to diagnose choriocarcinoma. Generally, a measurement of biological activity is expressed in IU, whereas measurement of the amount of a protein or of a specific protein structure is expressed in SI. In this case there would be a compelling reason to relate the measurement of hCG to a unit of biological activity, and the measurement of hCG-beta to an SI unit of quantity. Accordingly WHO has established a reference preparation for hCG (currently the fourth International Standard, with an assigned content of 650 IU/ampoule) (7) and a reference preparation for hCG-beta (currently the first WHO Reference Reagent for immunoassay of hCG beta subunit, with an assigned content of 0.88nmol/ampoule) (18). The former preparation was assigned a value based on bioassay, whereas the latter preparation had been extensively characterized by physicochemical and
immunological methods and calibrated in nanomol by amino acid analysis. Applying these considerations of the properties of biological analytes, and their measurement in the clinical situation allowed the WHO biological reference standard for hepatitis B surface antigen, assigned a value in arbitrary IU rather than in SI units, to be adopted by the medical devices sector of the European Commission as the standard required for the fulfilment of the so-called Common Technical Specifications (CTS) for in vitro diagnostic devices. The Common Technical Specification document supporting the European (IVD) Medical Devices Directive 98/79 EC is a legally binding document within the 25 countries of the European Union.
Where it is appropriate for a WHO biological reference standard to be calibrated in SI units, the principles outlined in ISO 17511 (8) should be followed. This will necessitate the existence and use of an appropriate single reference method and an assignment of uncertainty, derived from calibration data. Such a reference method should not be a biological assay because the factors that affect the results of such assays are not fully understood. Where they are used, SI units assigned to biological reference standards should be derived from, and traceable to, physicochemical procedures.
Note that IU are not the same as enzyme units (U) which is a measure of enzymatic activity