SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                                   -----------

                                  SCHEDULE 13D
                                 (Rule 13d-101)

             INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT
             TO RULE 13d-l(a) AND AMENDMENTS THERETO FILED PURSUANT
                                TO RULE 13d-2(a)

                                (Amendment No. 5)

                              Thoratec Corporation
________________________________________________________________________________
                                (Name of Issuer)

                      Common Stock, no par value per share
________________________________________________________________________________
                         (Title of Class of Securities)

                                   885175 30 7
________________________________________________________________________________
                                 (CUSIP Number)

                             Seth H. Hoogasian, Esq.
                                 General Counsel
                           Thermo Electron Corporation
                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046
                                 (781) 622-1000
________________________________________________________________________________
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                  June 29, 2004
________________________________________________________________________________
             (Date of Event Which Requires Filing of This Statement)

     If the filing  person has  previously  filed a statement on Schedule 13G to
report the  acquisition  that is the subject of this Schedule 13D, and is filing
this  schedule  because  of Rule  13d-1(e),  13d-1(f)  or  13d-1(g),  check  the
following box [ ].

     Note.  Schedules  filed in paper format shall include a signed original and
five copies of the schedule, including all exhibits. See Rule 13d-7(b) for other
parties to whom copies are to be sent.
                         (Continued on following pages)







                                                                                 

----------------------------------------                                       -------------------------------------

         CUSIP NO. 885175 30 7                          13D                            Page 2 of 6 Pages
----------------------------------------                                       -------------------------------------


-------- -----------------------------------------------------------------------------------------------------------

1.       NAMES OF REPORTING PERSONS
         I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)

         Thermo Electron Corporation
         04-2209186
-------- -----------------------------------------------------------------------------------------------------------

2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                                                (a)  [ ]
                                                                                                          (b)  [ ]
-------- -----------------------------------------------------------------------------------------------------------

3.       SEC USE ONLY
-------- -----------------------------------------------------------------------------------------------------------

4.       SOURCE OF FUNDS*

         SC
-------- -----------------------------------------------------------------------------------------------------------

5.       CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEM 2(d) or 2(e)                                                                                     [ ]
-------- -----------------------------------------------------------------------------------------------------------

6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         State of Delaware
---------------------------- -------- ------------------------------------------------------------------------------

     NUMBER OF               7.       SOLE VOTING POWER
      SHARES                              1,703,765
    BENEFICIALLY             -------- ------------------------------------------------------------------------------
   OWNED BY EACH
     REPORTING               8.       SHARED VOTING POWER
    PERSON WITH                           2,731,779
                             -------- ------------------------------------------------------------------------------

                             9.       SOLE DISPOSITIVE POWER
                                          4,435,544
                             -------- ------------------------------------------------------------------------------

                             10.      SHARED DISPOSITIVE POWER
                                             -0-
---------------------------- -------- ------------------------------------------------------------------------------

11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         4,435,544
---------- ---------------------------------------------------------------------------------------------------------

12.      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*                                [ ]

---------- ---------------------------------------------------------------------------------------------------------

13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         7.9%

---------- ---------------------------------------------------------------------------------------------------------

14. TYPE OF REPORTING PERSON*

           CO
---------- ---------------------------------------------------------------------------------------------------------

                      *SEE INSTRUCTIONS BEFORE FILLING OUT!







     Thermo  Electron  Corporation,  a  Delaware  corporation  ("the  "Reporting
Person"),  hereby  amends its  statement  on Schedule 13D relating to the common
stock,  no par value per share (the  "Common  Stock"),  of Thoratec  Corporation
(formerly known as Thoratec Laboratories Corporation),  a California corporation
(the "Company").

ITEM 2. Identity and Background.

     Item 2 is hereby amended and restated as follows:

     This  Amendment  is being filed by the  Reporting  Person  pursuant to Rule
13d-2 to reflect the change of information  previously  reported under Item 5 of
its Schedule 13D, as amended.

     The  principal  business  address  and  principal  office  address  of  the
Reporting Person is 81 Wyman Street, Waltham, Massachusetts 02454-9046.

     The Reporting  Person is a leading  provider of analytical  and  monitoring
instruments used in a broad range of applications,  from life sciences  research
to telecommunications to food, drug, and beverage production.

       Appendix A attached to hereto sets forth with respect to each executive
officer and director of the reporting Person the following information:

     (a)  name;
     (b)  business address;
     (c)  present  principal  occupation or employment  and the name,  principal
          business and address of any corporation or other organization in which
          such employment is conducted; and
     (d)  citizenship.
To the knowledge of the Reporting  Person,  there is no person who may be deemed
to be a controlling person of the Reporting Person.

     During the last five  years,  neither  the  Reporting  Person  nor,  to the
knowledge of the  Reporting  Person,  any  executive  officer or director of the
Reporting Person has been convicted in a criminal proceeding  (excluding traffic
violations or similar misdemeanors).

     During the last five  years,  neither  the  Reporting  Person  nor,  to the
knowledge of the  Reporting  Person,  any  executive  officer or director of the
Reporting  Person  has  been a party  to a civil  proceeding  of a  judicial  or
administrative  body of  competent  jurisdiction  which  resulted in a judgment,
decree  or final  order  enjoining  future  violations  of,  or  prohibiting  or
mandating activities subject to, Federal or State securities laws or finding any
violation with respect to such laws.

ITEM 5. Interest in Securities of the Issuer.

     Item 5 is hereby amended and restated in its entirety as follows:

     (a) As of the date of this Amendment,  the Reporting  Person owns 4,435,544
shares of Common Stock which represents 7.9% of the outstanding  Common Stock of
the Company,  based upon the total Common Stock  outstanding  as reported on the
Company's  Quarterly  Report on Form 10-Q for the  period  ended  April 3, 2004.

                                 (Page 3 of 6)


Information  with  respect to the  beneficial  ownership of the shares of Common
Stock of the other persons named in Item 2 is set forth in Appendix A. Except as
set forth in this Item 5, to the best knowledge of the Reporting Person, none of
the parties named in Item 2 owns any of the Company's Common Stock.

     (b) With  respect  to  2,731,779  shares  of  Common  Stock of the  Company
beneficially  owned by the Reporting  Person,  the  Reporting  Person shares the
power to vote all of such shares with the  Company  pursuant to the  Shareholder
Agreement,  as  more  fully  described  in Item 4  above.  With  respect  to the
remainder of the shares of Common Stock of the Company beneficially owned by the
Reporting  Person,  the  Reporting  Person  has the sole  power to vote all such
shares.  Subject  to the  terms  of the  Shareholder  Agreement,  as more  fully
described in Item 4 above, the Reporting Person has the sole power to dispose of
the shares of Common Stock  beneficially owned by it. With respect to the shares
of Common Stock of the Company  beneficially owned by the other persons named in
Item 2,  except as set forth in  Appendix A, each such person has the sole power
to vote all of such shares and the sole power to dispose of all of such shares.

     Item 1 sets forth the Company's name and state of incorporation  along with
the address of its  principal  business  and  principal  office.  The  Company's
principal business is the research, development,  manufacturing and marketing of
medical devices for circulatory support and vascular graft applications.  To the
best knowledge of the Reporting Person,  during the last five years, the Company
(i) has  not  been  convicted  in any  criminal  proceeding  (excluding  traffic
violations  or  similar  misdemeanors)  and  (ii)  was  not a  party  to a civil
proceeding of a judicial or  administrative  body of competent  jurisdiction  as
result  of which it was or is  subject  to a  judgment,  decree  or final  order
enjoining future violations of, or prohibiting or mandating  activities  subject
to,  Federal or State  securities  laws or finding any violation with respect to
such laws.

     (c) Except as  described  below in this Item 5 and in Item 4, which item is
incorporated  herein by  reference,  neither  the  Reporting  Person nor, to the
knowledge of the Reporting  Person,  any person named in Appendix A beneficially
has effected any transactions in Common Stock during the past 60 days.

     The  Reporting  Person has  affected the  following  open market sales with
respect to the Common Stock since the filing of Amendment  No. 4 to its Schedule
13D:


                                                                                     

---------------------------------------- -------------------------------------- --------------------------------------
                 Date                                   Amount                             Price Per Share
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                1,000,000                               $12.45
---------------------------------------- -------------------------------------- --------------------------------------
                3/10/04                                 250,000                                $12.99
---------------------------------------- -------------------------------------- --------------------------------------
                5/12/04                                 133,671                                $15.49
---------------------------------------- -------------------------------------- --------------------------------------
                5/14/04                                 18,326                                 $15.00
---------------------------------------- -------------------------------------- --------------------------------------
                6/2/04                                  98,003                                 $14.50
---------------------------------------- -------------------------------------- --------------------------------------
                6/9/04                                  62,600                                 $14.53
---------------------------------------- -------------------------------------- --------------------------------------
                6/10/04                                 110,675                                $14.51
---------------------------------------- -------------------------------------- --------------------------------------
                6/15/04                                 26,725                                 $14.42
---------------------------------------- -------------------------------------- --------------------------------------
                                 (Page 4 of 6)

---------------------------------------- -------------------------------------- --------------------------------------
                6/16/04                                 40,739                                 $14.52
---------------------------------------- -------------------------------------- --------------------------------------
                6/17/04                                 88,500                                 $14.38
---------------------------------------- -------------------------------------- --------------------------------------
                6/18/04                                 80,000                                 $14.37
---------------------------------------- -------------------------------------- --------------------------------------
                6/21/04                                 33,426                                 $14.33
---------------------------------------- -------------------------------------- --------------------------------------
                6/22/04                                 57,335                                 $14.22
---------------------------------------- -------------------------------------- --------------------------------------
                6/23/04                                 60,000                                 $14.24
---------------------------------------- -------------------------------------- --------------------------------------
                6/24/04                                 50,000                                 $14.23
---------------------------------------- -------------------------------------- --------------------------------------
                6/25/04                                 41,148                                 $14.22
---------------------------------------- -------------------------------------- --------------------------------------
                6/28/04                                 41,376                                 $14.20
---------------------------------------- -------------------------------------- --------------------------------------
                6/29/04                                 57,476                                 $14.14
---------------------------------------- -------------------------------------- --------------------------------------


     In addition to the foregoing, Mr. Theo Melas-Kyriazi,  an Executive Officer
of the  Reporting  Person,  has  affected the  following  open market sales with
respect to the Common Stock since the filing of Amendment No. 4 to Schedule 13D:


                                                                                     
---------------------------------------- -------------------------------------- --------------------------------------
                 Date                                   Amount                             Price Per Share
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                  6,500                                 $12.93
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                   800                                  $12.96
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                   300                                  $12.94
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                   200                                  $13.00
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                   100                                  $12.98
---------------------------------------- -------------------------------------- --------------------------------------
               12/16/03                                   100                                  $12.97
---------------------------------------- -------------------------------------- --------------------------------------
               12/17/03                                  8,500                                 $12.95
---------------------------------------- -------------------------------------- --------------------------------------
               12/17/03                                   200                                  $12.97
---------------------------------------- -------------------------------------- --------------------------------------


     (d) None.

     (e) Not applicable.

                                 (Page 5 of 6)



                                   SIGNATURES

     After  reasonable  inquiry and to the best of my  knowledge  and belief,  I
certify that the information  set forth in this statement is true,  complete and
correct.


                                                    Dated:  July 28, 2004

                                              THERMO ELECTRON CORPORATION


                                             By: /s/Theo Melas-Kyriazi
                                                ________________________________
                                                Name:   Theo Melas-Kyriazi
                                                Title:  Vice President and Chief
                                                        Financial Officer


                                  (Page 6 of 6)




                                   APPENDIX A

Thermo Electron Corporation

     The  name,  present  principal  occupation  or  employment  and  beneficial
ownership of shares of common stock of Thoratec  Corporation  (the "Company") of
each  director and executive  officer of Thermo  Electron  Corporation  ("Thermo
Electron") is set forth below.  Unless otherwise noted, all such individuals are
citizens of the United States.  Unless  otherwise noted, the business address of
each  executive  officer  and  director of Thermo  Electron is 81 Wyman  Street,
Waltham, Massachusetts 02454-9046.



                                                                                                     
Directors
-------------------------------------- ---------------------------------------- ---------------------------------------------
                                                                                   Beneficial Ownership of Shares of the
                                                                                            Company Common Stock
                                                                                ------------- ----------------- -------------
                                                                                                   Shares
                                                                                                 Underlying
                                                                                                  Options
                                                                                                Exercisable        Total
       Name/Present Principal                                                   Shares Held     Prior to May     Beneficial
      Occupation or Employment                    Business Address                Outright        31, 2004       Ownership
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
John L. LaMattina                      Pfizer Inc.                                       0                 0             0
Senior Vice President of Pfizer Inc.   50 Pequot Avenue
and the President of Pfizer Global     New London, Connecticut 06230
Research and Development
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Peter J. Manning
Director of Safety Insurance Group                                                       0                 0             0
Inc.
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Jim P. Manzi
Chairman of the Board of Thermo
Electron and Chairman of Stonegate                                                       0                 0             0
Capital
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Robert A. McCabe                       Pilot Capital Corporation
Chairman of Pilot Capital Corporation  2 East 73rd Street, 15th Floor                4,393                 0         4,393
                                       New York, New York 10021
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Robert W. O'Leary                      Valeant Pharmaceuticals International             0                 0             0
Chairman and Chief Executive Officer   3000 Hyland Avenue
of Valeant Pharmaceuticals             Costa Mesa, California 92626
International
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Michael E. Porter                      Harvard Business School                           0                 0             0
Bishop William Lawrence                Soldiers Field Road
University Professor at                Boston, Massachusetts 02163
Harvard Business School
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Elaine S. Ullian                       Boston Medical Center                             0                 0             0
President and Chief Executive          Talbot 1
Officer of Boston Medical Center       One Boston Medical Center Place
                                       Boston, Massachusetts 02118
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Marijn E. Dekkers(1)                                                                     0                 0             0
President, Chief Executive Officer
and Director of Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------


(1) Mr. Dekkers is a citizen of The Netherlands.

                                  (Page I - 1)





                                                                                                     

Executive Officers Who are Not Directors

         No person is a controlling stockholder of Thermo Electron.

-------------------------------------- ---------------------------------------- ---------------------------------------------
                                                                                   Beneficial Ownership of Shares of the
                                                                                            Company Common Stock
                                                                                ---------------------------------------------
                                                                                ------------- ----------------- -------------
       Name/Present Principal                     Business Address              Shares Held        Shares          Total
                                                                                                 Underlying
                                                                                                  Options
                                                                                                Exercisable
                                                                                                Prior to May     Beneficial
      Occupation or Employment                                                    Outright        31, 2004       Ownership
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Guy Broadbent(2)                                                                         0                 0             0
Vice President of Thermo Electron,
President, Optical Technologies
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Marc N. Casper                                                                           0                 0             0
Senior Vice President of
Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Seth H. Hoogasian                                                                        0                 0             0
Vice President, General Counsel and
Secretary of Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Peter E. Hornstra                                                                        0                 0             0
Corporate Controller and Chief
Accounting Officer of Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Theo Melas-Kyriazi(3)                                                                2,975                 0         2,975
Vice President and Chief Financial
Officer of Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Stephen G. Sheehan                                                                       0                 0             0
Vice President, Human Resources of
Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Peter M. Wilver                                                                          0                 0             0
Vice President, Financial Operations
of Thermo Electron
------------------------------------------------------------------------------- ------------- ----------------- -------------

------------------------------------------------------------------------------- ------------- ----------------- -------------
All directors and current executive officers as a group                              7,368                 0         7,368
------------------------------------------------------------------------------- ------------- ----------------- -------------


(2) Mr. Broadbent is a citizen of the United Kingdom.

(3) Mr. Melas-Kyriazi is a citizen of Greece.


                                  (Page I - 2)