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Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation— The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH (“X4 Austria”), which is incorporated in Vienna, Austria, and X4 Therapeutics, Inc. All intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates— The preparation of the Company’s consolidated financial statements in conformity with U.S. Generally Accepted Accounting Principles (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, and the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. As of the date of issuance of these consolidated financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s consolidated financial statements.
Goodwill	Goodwill— There were no triggering events during the three months ended March 31, 2024 that necessitated an interim impairment test of goodwill.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Standards In November 2023, the Financial Accounting Standards Board (“FASB”) issued ASU 2023-07, Segment Reporting (Topic 326) Improvements to Reportable Segment Disclosures (“ASU 2023-07”). Among other disclosure enhancements, ASU 2023-07 requires that entities with one reportable segment, such as the Company, disclose general information for its reportable segment, such as the title and position of the individual identified as the Chief Operating Decision Maker (“CODM”), which for the Company is the Chief Executive Officer, the types of products and services provided by the reportable segment, the measure of profit or loss reviewed by the CODM to evaluate performance of the reportable segment and other financial results such as interest income, interest expense and depreciation associated with the reportable segment. The amendments in ASU 2023-07 will become effective for the Company in its consolidated financial statements as of and for the three years ending December 31, 2024 and must be adopted retrospectively. Although the Company continues to evaluate the potential impact of ASU 2023-07, the Company does not believe that the adoption of ASU 2023-07 will have a material impact on its consolidated financial statement when adopted. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740) Improvements to Income Tax Disclosures (“ASU 2023-09”). The amendments in ASU 2023-09 require that entities on an annual basis disclose specific categories in the income tax rate reconciliation and provide additional information for reconciling items if the effect of those reconciling items that exceed a certain threshold. ASU 2023-09 will also require more disaggregated disclosures related to income taxes paid. The amendments in ASU 2023-09 will become effective for the Company in its December 31, 2024 consolidated financial statements. Although the Company continues to evaluate the impact of ASU 2023-09, the Company expects that these amendments will require further disclosures in the tax footnote of its annual consolidated financial statements and will not have a material impact on its consolidated financial states when adopted.
Research and Development Expense, Policy

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Disclosure of accounting policy regarding (1) the principles it follows in consolidating or combining the separate financial statements, including the principles followed in determining the inclusion or exclusion of subsidiaries or other entities in the consolidated or combined financial statements and (2) its treatment of interests (for example, common stock, a partnership interest or other means of exerting influence) in other entities, for example consolidation or use of the equity or cost methods of accounting. The accounting policy may also address the accounting treatment for intercompany accounts and transactions, noncontrolling interest, and the income statement treatment in consolidation for issuances of stock by a subsidiary.

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Disclosure of accounting policy for goodwill. This accounting policy also may address how an entity assesses and measures impairment of goodwill, how reporting units are determined, how goodwill is allocated to such units, and how the fair values of the reporting units are determined.

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