The drug interaction databases classify this as a moderate interaction, noting that "as you lose weight during treatment with phentermine, your dosage requirement of tirzepatide or any other diabetic medications may change." This is not a minor technicality
[DOI] [PMC free article] [PubMed] [Google Scholar] 60.Wu J., Lipinski M.M
The standard pharmaceutical protocol uses 0.25, 0.50, 1.0, 1.7, and 2.4mg dose levels
So there may be a little prophylactic ultrasound being done just to be sure, especially if theres any family history of thyroid problems, which are very common and are usually not related to thyroid cancer
If you experience regular vomiting, speak to your prescriber about timing your medications or managing the nausea