As in the United States. You would write U.S.
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You can use either, though do use capitals: (U.S. or U.S.A.) or (US or USA). The modern trend is not to use fullstops between the letters.
US or USA
northern US N. US or No. US (not recommended- "No." also means number)
i abbreviate it as advs. You could also abbreviate it as adv.
HTS or HGTS as per the US Postal service website.
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In medical terms you can abbreviate it to "Tx" like you abbreviate History to "Hx"
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You cannot abbreviate the number.
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