From an Altmetric.com Blog Post:
We’re proud to announce today that Altmetric has begun displaying a wealth of online attention paid to clinical trial study records from ClinicalTrials.gov, the world’s largest registry of clinical trials. Operated by the United States National Library of Medicine (NLM) at the National Institutes of Health, ClinicalTrials.gov holds registrations from about 200,000 trials from more than 170 countries in the world.
Currently, online attention collected from our various sources since 2014 has been matched to over 10,000 study records from ClinicalTrials.gov; each of these has its own Altmetric details page.
[Our emphasis] Users of the Altmetric Explorer and Altmetric for Institutions can quickly find all tracked ClinicalTrials.gov study records by entering “ClinicalTrials.gov” into the journal filter on the left sidebar. Within the Explorer and AFI, all users can browse through the entire collection of clinical trial study records and read through all the mentions.
[Our emphasis] At the moment, it isn’t possible to use the Altmetric Bookmarklet on ClinicalTrials.gov pages, but we do plan to do this in future. For now, it’s easiest and quickest to access the data using the Explorer/Altmetric for Institutions and our API.