I went to the dermatologist, but it was closed today.
I went to the dermatologist, but they were closed today.
「皮膚科」は英語でdermatologistと言います。なので、「皮膚科に行ったら今日はやってなかった」を英語で表現するとI went to the dermatologist, but it was closed today.あるいはI went to the dermatologist, but they were closed today.
It was closed today.→Itは「皮膚科のクリニック、お店そのもの」をさします。
They were closed today.→Theyは「そこで働いている人」をさします。
ニュアンスの違いはこれほどしかなく、この例文の皮膚科に限らず、基本どのシチュエーションでもどちらを使ってもOKです。
例文
I went to the restaurant, but it was closed today.
レストランに行ったけど、今日はやってなかった。
I went to the bakery, but they were already closed for the day.
パン屋さんに行ったけど、もうやってなかった。
I went to the dermatologist but they weren’t open today.
"皮膚科に行ったら今日はやってなかった"
- I went to the dermatologist but they weren’t open today.
- I went to see the dermatologist but they were closed today.
皮膚科は今日やってなかった
- The dermatology clinic was closed.
- The dermatologist wasn’t there today.
- The dermatologist was off today.
- The dermatology clinic wasn’t open.
アメリカ合衆国 
