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Cover
Aug. 06, 2021
Jun. 28, 2021
Cover [Abstract]    
Document Type 8-K  
Document Period End Date Aug. 06, 2021  
Entity Registrant Name X4 PHARMACEUTICALS, INC.  
Entity Incorporation, State or Country Code DE  
Entity File Number 001-38295  
Entity Tax Identification Number 27-3181608  
Entity Address, Address Line One   61 North Beacon Street,
Entity Address, Address Line Two   4th Floor
Entity Address, City or Town   Boston,
Entity Address, State or Province   MA
Entity Address, Postal Zip Code   02134
City Area Code 857  
Local Phone Number 529-8300  
Written Communications false  
Soliciting Material false  
Pre-commencement Tender Offer false  
Pre-commencement Issuer Tender Offer false  
Title of 12(b) Security Common Stock, par value $0.001 per share  
Trading Symbol XFOR  
Security Exchange Name NASDAQ  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Central Index Key 0001501697  
Amendment Flag false