Researcher responsible for the development of levothyroxine

I don't know if this would help, but maybe try searching for the developer of synthetic thyroid hormone, rather than by a specific name. Before there was synthetic thyroid hormone (such as levothyroxine), they used dessicated animal thyroid to supplement (some people still use that, but there are some issues with that, which I won't get into here).

Some other words in combination with "synthetic thyroid hormone" that might give you some results besides "developer" could be "origin," "creation," "first use," etc.

Hope that helps.