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. 4)

                              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
                                 (781) 622-1000
                           Thermo Electron Corporation
                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046
                   ------------------------------------------
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                September 9, 2003
                   ------------------------------------------
             (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 5 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 SHARES        7.       SOLE VOTING POWER
BENEFICIALLY OWNED BY EACH                   3,953,765
   REPORTING PERSON WITH
                             -------- ------------------------------------------------------------------------------
                             8.       SHARED VOTING POWER 2,731,779

                             -------- ------------------------------------------------------------------------------
                             9.       SOLE DISPOSITIVE POWER
                                             6,685,544
                             -------- ------------------------------------------------------------------------------

                             10.      SHARED DISPOSITIVE POWER
                                             -0-
---------------------------- -------- ------------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
           6,685,544

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

---------- ---------------------------------------------------------------------------------------------------------
13.        PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 12.0%

---------- ---------------------------------------------------------------------------------------------------------
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 of 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 6,685,544
shares of Common Stock which represents 12.0% 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 June 28, 2003. Of
the 6,685,544 shares of Common Stock currently owned by the Reporting Person,

                                  (Page 3 of 5)


5,010  shares of Common  Stock are  subject to options  to acquire  such  shares
granted by the  Reporting  Person  pursuant to its director  and employee  stock
option plans. 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 i
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 sale with
respect to the Common Stock since the filing of Amendment  No. 3 to its Schedule
13D:

--------------------------------------------------------------------------------
      Date                   Amount                             Price Per Share

---------------------------------------- ---------------------------------------
     9/9/03                 1,000,000                               $16.83
---------------------------------------- ---------------------------------------

     (d) None.

     (e) Not applicable.

                                 (Page 4 of 5)




                                   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: October 24, 2003

                                                THERMO ELECTRON CORPORATION


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


                                 (Page 5 of 5)




                                   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
                                                                                                  Prior to         Total
       Name/Present Principal                                                   Shares Held     September 9,     Beneficial
      Occupation or Employment                    Business Address                Outright          2003         Ownership
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
John L. LaMattina                      Pfizer, Inc.                                      0                 0             0
Executive Vice President of Pfizer     50 Pequot Avenue
Global Research and Development        New London, Connecticut 06230
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Peter J. Manning
Director of Papa Gino's                                                                  0                 0             0
Holding Corp.
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Jim P. Manzi                                                                             0                 0             0
Chairman of Stonegate Capital
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Robert A. McCabe                       Pilot Capital Corporation
Chairman of Pilot Capital Corporation  444 Madison Avenue
                                       Suite 2103                                    6,393                 0         6,393
                                       New York, New York 10022
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Robert W. O'Leary                      ICN Pharmaceuticals, Inc.                         0                 0             0
Chairman and Chief Executive Officer   3000 Hyland Avenue
of ICN Pharmaceuticals, Inc.           Costa Mesa, California 92626
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
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
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Richard F. Syron
Executive Chairman and
Chairman of the Board of                                                                 0                 0             0
Thermo Electron
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
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
                                                                                                September 9,     Beneficial
      Occupation or Employment                                                    Outright          2003         Ownership
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Guy Broadbent(2)                                                                         0                 0             0
Vice President of Thermo Electron,
President, Optical Technologies
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Marc Casper                                                                              0                 0             0
Vice President of Thermo Electron,
President, Life and Laboratory
Sciences
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
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
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Barry S. Howe                                                                            0                 0             0
Vice President of Thermo Electron,
President, Measurement and Control
-------------------------------------- ---------------------------------------- ------------- ----------------- -------------
Theo Melas-Kyriazi(3)                                                                2,963            16,700        19,663
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                              9,356            16,700        26,056
------------------------------------------------------------------------------- ------------- ----------------- -------------

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

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


                                  (Page I - 2)