[Mariana (OP)]

Pravin asks:

If proteins are broken down into amino acids before gut absorption, how do medicines that are proteins get into the bloodstream intact? For example, if I ingest thyroid hormone, how will it get to the bloodstream intact?

Do you know the answer?

[chiralSPO]

Thyroid hormones are not made of proteins, and so can avoid that problem.

Some drugs are polypeptides (a class that includes proteins), and are often administered by injection (circumventing the gut, which would surely destroy them)

[evan_au]

There are various ways that a medication can avoid being broken down in the stomach:
- Some drug absorption happens through the cheeks or under the tongue
- You can design the drug to be stable in the acidic conditions of the stomach
- If the stomach breaks down 90% of the drug, just make the dose 10x larger!
- You can encase the drug in a capsule, which dissolves only after the capsule has passed through the stomach
- Some pills have an "enteric coating" which does a similar thing
- You can administer the drug rectally. These drugs do not pass through the liver (the liver breaks down a lot of drugs too).

The challenges are even greater for oral medications intended for the brain and central nervous system, since the blood/brain barrier blocks many medications that successfully navigate the stomach and liver...

See: https://en.wikipedia.org/wiki/Route_of_administration#Enteral/gastrointestinal

[evan_au]

Some drugs can be administered as a patch.

Researchers are looking at various ways of getting insulin into the bloodstream to treat diabetes.
https://en.wikipedia.org/wiki/Insulin_(medication)#Methods_of_administration