I am not a medical expert and will not attempt to translate your sentence. I can, however, remark on the grammatical interpretations of のみ and 唯一 in your sentence. In effect, there are two ways to interpret their use, and you must choose which one is correct based on their implied medical implication.

Firstly, in both cases のみ refers to D欠乏症 as the sole agent.

Secondly, the use of 唯一 can be redundant as you suggest, or it can have a meaning all of its own.

Case One (No Redundancy):

「D欠乏症のみが」「PTH作用低下以外に唯一低Ca血症を引き起こす内分泌疾患」である。

In this phrase 唯一 refers to the mere effect of 引き起こす

Case Two (Redundancy):

「D欠乏症のみがPTH作用低下以外に唯一低Ca血症を引き起こす」内分泌疾患である。

In this case 唯一 reinforces the idea that D欠乏症 is the only active agent.

Concluding remark: In general, I give authors the benefit of the doubt and assume that they know their own language better than I -- in effect, that they would not be redundant unless there were an important need for emphasis.