uisaga che-oni naeryeogamyeon naeilbuteo hakgyoe gado doendago haesseo.

Because the sentence contains a main clause and an embedded clause.

• 의사가 = the doctor is the subject of 했어 (said)
• 체온이 = body temperature is the subject of 내려가면 (if it goes down)

So the structure is roughly:

• [체온이 내려가면] = if the temperature goes down
• [의사가 ... 했어] = the doctor said ...

It can feel strange in English because English often avoids repeating subject marking so clearly, but in Korean this is very normal.

내려가면 comes from:

• 내려가다 = to go down / to decrease
• -면 = if / when

So:

• 체온이 내려가다 = the body temperature goes down
• 체온이 내려가면 = if the body temperature goes down

In this sentence, 내려가다 is being used in the sense of decrease, not literal physical downward movement.