"^F^bSJy.mr"}       Preparation  of  Thyroid  Extract.  57 
been  made  in  this  laboratory  for  the  iodine  content  and  it  has  been 
found  that  the  products  from  various  firms  differ  widely  in  the  con- 
tent of  iodine,  and  also  that  the  same  preparations  vary  from  time  to 
time. 
The  precise  relation  which  iodine  has  to  the  physiology  of  the 
thyroid  has  been  a  subject  of  much  discussion  and  in  a  recent  Bulle- 
tin from  the  Hygienic  Laboratory  Hunt  and  Seidell  have  reviewed 
the  arguments  pro  and  con  in  regard  to  this  matter,  and  have  given 
the  results  of  a  long  series  of  experiments  based  upon  a  new  method 
to  show  that  there  is  a  very  close  relation  between  iodine  content 
and  physiological  activity.  The  precise  function  of  the  gland  need 
not  be  called  in  question  in  discussing  this  point.  We  know  that 
the  thyroid  gland  has  a  very  marked  selective  absorption  for  iodine. 
In  this  laboratory  we  have  made  many  analyses  to  determine  the 
iodine  content  of  liver,  kidney,  and  muscles  taken  from  animals  to 
which  large  quantities  of  potassium  iodide  had  recently  been  given 
and  have  not  found  the  slightest  trace  of  it,  while  in  the  same  ani- 
mals the  thyroid  gland  may  have  had  its  iodine  content  increased 
by  200-600  per  cent.  In  vitro  there  is  no  more  difficulty  in  iodizing 
a  proteid  from  these  other  tissues  than  from  the  thyroid,  so  that 
Blum's  *  belief  that  proteids  artificially  iodized  in  vitro  should  be 
considered  identical  to  those  formed  in  vivo  in  the  thyroid  certainly 
has  no  justification. 
(Blum  has  maintained  the  theory  that  the  function  of  the  thyroid 
is  to  detoxicate  certain  metabolic  toxins  by  combining  iodine  with 
them,  and  in  part  bases  the  theory  upon  the  experimental  finding 
that  thyroid  extract  which  has  been  saturated  with  iodine  in  vitro 
no  longer  has  the  same  physiological  action  that  it  does  before  the 
artificial  saturation  with  iodine.  From  this  finding  he  reasons  that 
the  addition  of  iodine  destroys  the  toxic  properties  of  the  substances 
brought  to  the  gland  in  the  circulation,  and  that  the  more  completely 
this  is  accomplished  the  less  toxic  the  products  are.  The  fallacy 
of  this  argument  is  to  be  found  in  the  fact  that  iodizing  of  a  proteid 
in  vitro  is  a  drastic  chemical  process  in  no  way  to  be  compared  with 
the  physiological  action  of  the  thyroid  gland.) 
No  artificial  product  has  ever  been  prepared  which  has  the  same 
effect  upon  metabolism,  myxoedema,  and  cretinism  that  is  obtained 
by  iodized  proteid  from  the  thyroid  gland.  The  discussion  of  the 
pharmacological  action  and  therapeutic  value  of  thyroid  prepara- 
tions need  not  involve  us  in  discussion  of  the  function  of  the  gland. 
