When entering the U.S. in-transit to a foreign destination, you will be required to clear U.S. Customs Border Protection and Immigration Customs Enforcement. If you have "negotiable monetary instruments" (i.e. currency, personal checks (endorsed), travelers checks, gold coins, securities or stocks in bearer form) valued at $10,000 or more in your possession a "Report of International Transportation of Currency or Monetary Instruments" form FinCEN 105 must be submitted to a CBP officer upon your entry into the United States.
Monetary instruments that are made payable to a named person but are not endorsed or which bear restrictive endorsements are not subject to reporting requirements, nor are credit cards with credit lines of over $10,000. Gold bullion is not a monetary instrument for purposes of this requirement. The requirement to report monetary instruments on a FinCEN 105 does not apply to imports of gold bullion.
Failure to declare monetary instruments in amounts of or over $10,000 can result in its seizure.
You can obtain the form in advance and download it from here FinCEN 105, or a CBP officer can give it to you.