28
devoid of physiological activity. Hence an iodoprotein containing only these iodoamino acidsis
likely to be also physiologically inactive. The
reason these individuals have such an iodoprotein
in the blood is not clear. The data on norma! controls from the Eastern United States, who showed
0.80 ug% iodoprotein iodine in their serum, sug-
gest that it is a normal, albeit minor, constituent.
The method of chromatography employed ts such
that well under 5% (or 0.2 pg%) of serum thy-
roxine iodine appears in the unretarded or iodoprotein fraction. Therefore, the finding of iodo-
protein does not appear to be a methodologic
artifact.
Morerecently, with the developmentof thyroid
abnormalities in the exposed Marshallese (to be
described), it was possible to examine serum iodoprotein levels in cases with thyroid hypofunction.
These data are presented in Table 16. It seems
likely that the source of the iodoprotein waslargely
extrathyroidal, since the levels of iodoprotein were
Table 16
to Thyroid Function
No.
Total
iodine,
ug%
PBI, 7, iodine, -
ug%
gto
lodoprotem
iodine,*
ugx
Hypothyroid
3
3.2
5
63
3.1
3.1
10.
1.8
19
{2.2)
(1.3)
(1.2)
Thyroidectomized**
17
1.8
<0.5
(>1.3)
64
69
5.0
5.7
2.0
1.7
2.9
(4.0)
2]
13
<0.5
(>0.8)
t-Thyroxine Treated t
34
59
68
10.8 [9.1]
8.6 {8.2}
11.8 [7.2]
6.5 [4.6]
4.8 [5.3]
5.8 [4.6]
cases which had been on suppressive thyroxine
therapy, (b) in cases with atrophic glands due to
radiation (subjects No. 3 and No. 5), and (c) in
thyroidectomized cases. The source of the :odoprotein is not known. The previously reported finding
of high plasma proteins,* particularly gamma
globulins, in the Marshalleseis of interest but, may
be an unrelated phenomenon. It will be important
to see in future studies whether the iodoprotein
can be labeled with radioiodine.
The data on urine iodine show values in the
normalrange. In general, it had been expected
that individuals living close to the sea and eating
seafood and fish would show relatively higher
iodine intake. The inhabitants of the Marshall
Islands have fish as one of their main sources of
animal protein. Furthermore, these people are
constantly exposed to sea spray, since theisland
at its widest is about % mile across, and its highest
point is =20 ft above high tide.
The data on urinary iodine were used with the
results obtained with '**I studies to calculate the
amount of iodine secreted daily by the thyroid.
The value 76 pg/day is somewhat higherthan the
Serum lodoprotein Levels in Relation
Subject
near the normally high Marshallese values (a) in
3.2 [4.5]
3.8 [2.9]
6.0 [2.5]
*lodoprotein levels in parentheses represent the differ-
ence between PBI and 7,levels. The others were measured directly by the column method.
**.-Thyroxine stopped several weeks before sampling.
*Treated with c-thyroxine, 0.2 mg per day, for 6
months. Values in brackets are determination made prior
lo Starting thyroxine treatment (1965).
57 pg/day found by Stanbury et al.** or 58 ug/day
found by Freinkel and Ingbar** but closer to the
value 70 pg/day proposed by Riggs.?’ Unfortunately, nothing is known about therate of turnover of the serum iodoprotein. If it has roughly the
same rate of degradation and the same volume of
distribution as thyroxine andit is assumed to have
its origin in the thyroid gland, then one would expect the Marshallese thyroids to secrete organic
iodine proportional to the level of organic iodine
in their serum. Addition of iodothyroxine values
to iodoprotein iodine levels for both Marshallese
and Americans and multiplication of the ratio by
the best value for iodine secreted by normal!
Americansresults in
2.22 +4.53
0.8043.76" 58 = 86 pe/day.
This agrees fairly well with the 76 pg/day calculated independently from urine and radioiodine
studies and is compatible with the clinical picture
of a euthyroid status in spite of an elevated PBI
and elevated thyroid iodine secretion rate. However, as noted above, it seems somewhat morelike-
ly that the serum iodoprotein is not of thyroidal
origin.