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Cover
Mar. 04, 2021
Jan. 08, 2021
Cover [Abstract]    
Document Type 8-K  
Document Period End Date Mar. 04, 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