<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-04-07</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0000859737</issuerCik>
        <issuerName>HOLOGIC INC</issuerName>
        <issuerTradingSymbol>HOLX</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001400568</rptOwnerCik>
            <rptOwnerName>Wendell Amy McBride</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>250 CAMPUS DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MARLBOROUGH</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>01752</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

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            </securityTitle>
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                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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                </transactionShares>
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                    <footnoteId id="F2"/>
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                <directOrIndirectOwnership>
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                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
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            <expirationDate>
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                </underlyingSecurityTitle>
                <underlyingSecurityShares>
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            <transactionTimeliness></transactionTimeliness>
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                </underlyingSecurityTitle>
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            </underlyingSecurity>
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        <derivativeTransaction>
            <securityTitle>
                <value>Non-qualified Stock Option (Right to Buy)</value>
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            </conversionOrExercisePrice>
            <transactionDate>
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            </transactionDate>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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                </transactionShares>
                <transactionPricePerShare>
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            <expirationDate>
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            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
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                </underlyingSecurityTitle>
                <underlyingSecurityShares>
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            </underlyingSecurity>
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        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Non-qualified Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
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            </conversionOrExercisePrice>
            <transactionDate>
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            </transactionDate>
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                <transactionFormType>4</transactionFormType>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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            <expirationDate>
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            <underlyingSecurity>
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            <transactionDate>
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                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <exerciseDate>
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            <expirationDate>
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            <underlyingSecurity>
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                </underlyingSecurityTitle>
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        <derivativeTransaction>
            <securityTitle>
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            <transactionDate>
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            <transactionTimeliness></transactionTimeliness>
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            <securityTitle>
                <value>Non-qualified Stock Option (Right to Buy)</value>
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                <transactionPricePerShare>
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            <securityTitle>
                <value>Non-qualified Stock Option (Right to Buy)</value>
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                <transactionShares>
                    <value>4536</value>
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                <transactionPricePerShare>
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            <expirationDate>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
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                <directOrIndirectOwnership>
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            <securityTitle>
                <value>Non-qualified Stock Option (Right to Buy)</value>
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                <value>64.36</value>
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            <transactionDate>
                <value>2026-04-07</value>
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                <transactionShares>
                    <value>5535</value>
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                    <value>D</value>
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            <expirationDate>
                <value>2035-02-26</value>
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                <underlyingSecurityTitle>
                    <value>Common Stock</value>
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    </derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the Agreement and Plan of Merger, dated as of October 21, 2025 (the &quot;Merger Agreement&quot;), by and among Hologic, Inc. (&quot;Hologic&quot; or &quot;Company&quot;), Hopper Parent Inc., a Delaware corporation (&quot;Parent&quot;), and Hopper Merger Sub Inc., a Delaware corporation and wholly owned subsidiary of Parent (&quot;Merger Sub&quot;), Merger Sub merged with and into the Company (the &quot;Merger&quot;), with the Company surviving the Merger as a wholly owned subsidiary of Parent.  At the effective time of the Merger (the &quot;Effective Time&quot;), each share of Hologic common stock, par value $0.01 (&quot;Company Common Stock&quot;), was converted into the right to receive (x) $76.00 per share in cash, without interest (the &quot;Cash Consideration&quot;) and (y) one (1) contingent value right, which represents the right to receive up to $3.00 in cash, when and if payable (each, a &quot;CVR&quot;) (the consideration contemplated by clauses (x) and (y), together, the &quot;Merger Consideration&quot;).</footnote>
        <footnote id="F2">At the Effective Time, each time-vesting restricted stock unit award (a &quot;Company RSU Award&quot;) held directly by the reporting person was converted into the right to receive the Merger Consideration in respect of each share of Company Common Stock underlying the Company RSU Award. As a result of the Merger, the reporting person no longer beneficially owns, directly or indirectly, any shares of Company Common Stock.</footnote>
        <footnote id="F3">For Footnote (3), see Remarks below.</footnote>
    </footnotes>

    <remarks>(3)  Pursuant to the Merger Agreement, each outstanding option to purchase shares of Company Common Stock (a &quot;Company Option&quot;) with an exercise price per share less than the Cash Consideration was cancelled and converted into the right to receive (i) an amount in cash equal to the product of (A) the number of shares of Company Common Stock subject to such Company Option, multiplied by (B) the excess of the Cash Consideration over the exercise price per share of the Company Option, and (ii) one CVR with respect to each share.  Each outstanding Company Option with an exercise price per share equal to or greater than the Cash Consideration and less than the sum of the Cash Consideration and $3.00 was cancelled and converted into the right to receive one CVR with respect to each share of Company Common Stock subject to such Company Option, payment in respect of which will be net of the excess of the applicable exercise price per share of the Company Option over $76.00.  Each outstanding Company Option with an exercise price per share of Company Common Stock equal to or greater than the sum of the Cash Consideration and $3.00 was cancelled for no consideration.</remarks>

    <ownerSignature>
        <signatureName>/s/ Mark W. Irving, attorney-in-fact for Ms. Wendell</signatureName>
        <signatureDate>2026-04-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
