Brief Answer:
Thyroid

Detailed Answer:
It is quite uncommon overall. In 20 year olds and females too, it is an extremely uncommon condition.

Your sister's swollen thyroid needs to comprehensively evaluated by a qualified endocrinologist in-person.

When I see someone like her in my practice I order the following tests in addition to a thorough physical examination :

CBC
Electrolytes
Calcium
Glucose
Creatinine
SGOT
SGPT
TSH

A thyroid Ultrasound is strongly considered

Further assessments if necessary are based on results of the above tests

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Brief Answer:
Follow up

Detailed Answer:
Thyroid cancer is typically managed by endocrinologists in USA in conjunction with Endocrine / thyroid surgeons and medical/ radiation and surgical oncologists, as required depending on the stage of the cancer.
So you see it is team work.

But at this point, diagnosis can be made confidently by an endocrinologist. However there is a systematic approach for this that entails:
Physical examination
Laboratory tests
Ultrasound
Consideration for fine needle aspiration biopsy and cytology

Merely sharing cursory information on an online forum such as this is not enough to make a diagnosis of something as serious as cancer.

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Brief Answer:
Second follow up

Detailed Answer:
I cannot commit to an answer without having the opportunity to glean detailed information by a comprehensive approach including examination and testing

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