Grammar rules for parellelism in comparisons and variations according to verb placement

Median serum thyroxine was significantly decreased in patients with a suppressed TSH compared to those with a normal TSH.

What does "those" refer to?

Median serum thyroxine is one value (level) at the midpoint of the levels of a group of patients. Got it. But in the comparison group, you use those. It's ambiguous to me, because I presume "those" refers to the median serum thyroxine (level) in the unsuppressed patients, which should be a single value as well.

To me, it has nothing to do with verb placement, but rather to the referent. "Those" refers not to patients (plural) but a median level (singular). Restated:

Median serum thyroxine [level, singular] was significantly decreased in patients [plural] with a suppressed TSH compared to those [levels, plural? patients, plural?] with a normal TSH.

Compare to #2:

Median serum thyroxine [level, singular] was significantly decreased in patients [pl] with a suppressed TSH compared to that [level, singular] in patients [pl] with a normal TSH.

All those match up. Your pronoun/determiner matches in number to your first noun (level).

Compare to #3:

Median serum thyroxine [level, singular] in patients [pl] with a suppressed TSH was significantly decreased compared to that [level, singular] in patients [pl] with a normal TSH.

There isn't anything tricky about the verbs. If you referred to the hormone level instead of leaving that floating in the ether somewhere, I don't think you would run into the problem you have grappled with.

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