I knewrealized a long time ago, that my parents will need to be taken care of a little bit earlier than expected.

"knew a long time ago" implies that something has been forgotten.

you could also say: I have known for a long time that my parents...

I started working only 2 years ago, and I need to plan ahead in the future, just in case my dad or mom gets worse, (I hope and pray they don't). In order to accomplish this, not only do I need to upgrade their health plan, but I also need to check more often with their doctors.

I think that "not only______but _______" sentences usually say the subject in both spaces when the verb/action changes.

For example:

1. In order to accomplish this, not only do I need to upgrade their health plan, but also their exercise schedule.

There both parts of the "not only...but" are about upgrading something, so the subject isn't repeated.

My mom has a dysfunction in itsher thyroid glands.

"the" could be accepted instead of "her," but "her" is more natural

She doesn't hasve a specialized doctor thatwho can treat her correctly, due to hiser subsidized health plan, itwhich is good for general purposes but lacks the specialists that we need right now.

"doesn't" changes it so that "has" needs to be "have" Example:

He has a job.
He doesn't have a job.

"who" is sometimes preferred when discussing people

"which" is helpful to use when referring to something just introduced that is being referred to already for clarification

I'll make an appointment with a doctor in our area, O once I find a reputable endocrinologist.

The comma is unnecessary because that part is a dependent clause that is necessary in the sentence.

I knewrealized a long time ago, that my parents willould need to be taken care of a little bit earlier than expected.

1. “Knew” technically works, but it sounds odd, using the verb “realize” sounds better
2. Additionally, you would also use “would” instead of “will” here, as “would” is used when talking about future scenarios from a past perspective, or hypothetical scenarios

I started working only 2 years ago, and I need to plan ahead in the future, just in case my dad or mom gets worse, (I hope and pray they don't). In order to accomplish this, not only do I need to upgrade their health plan, but also check more often with doctors. more often.

1. The phrase “Not only do I ___” needs to the have the “do”, I’m not sure why, that’s just how it’s said in English
2. The rest is minor grammar errors, and I adjusted the end of the last sentence to make it sound more natural

My mom has a dysfunction in itsher thyroid glands.

The thyroid glands belong to your mom, not an animal or an object, so you need to use “her”

She doesn't hasve a specialized doctor that can treat her correctly, due to hiser subsidized health plan, it is good for general purposes but lacks the specialists that we need right now.

I’m assuming that you were talking about your mom’s health plan, so you’d use “her”. “His” is for men, “her” is for women.

I'll make an appointment with a doctor in our area, Oonce I find a reputable endocrinologist.

I started working only 2 years ago, and but I need to plan ahead in the future, just in case my dad or mom gets worse, (I hope and pray they don't). In order to accomplish this, I not only I need to upgrade their health plan, but also check more often with doctors.

I just changed minor grammatical things - you wouldn't need a comma before brackets as the brackets act a bit like commas, you didn't have a space after the full stop but that seems likely to have been a typo and word order needs to be "I not only" or "not only do I"

My mom has a dysfunction in itsher thyroid glands.

In English, when we refer to parts of someone's body, we say them in a possession way. E.g., she has pain in her head.

She doesn't hasve a specialized doctor that can treat her correctly, d. Due to this subsidized health plan, it is good for general purposes but lacks the specialists that we need right now.

A couple of typos and just "have" instead of "has" and that's just verb endings. I have made it into two sentences as the "Due to [...] plan" bit is a subordinate clause and so needed to be separated from the unrelated main clause

I'll make an appointment with a doctor in our area, Oonce I find a reputable endocrinologist.