Serum Pratein-Bound Iodine—Rall and Conard
asymptotic uptake is normal or slightly elevated. These data together with the urinary
iodine values, may be used to calculate the
average daily secretion of thyroid hormone,
assuming
steady
state
conditions,
by
the
formula S$ = > where
5
amount of iodine secreted by the thyroid
(micrograms per day)
U = fractional thyroid uptake of iodine
E = urinary iodine (microgramsper day)
Using E = 105 ug. per day and U = 0.42,
an average value for 5 may be calculated to
be 76 ug. of iodine per day. This value is somewhat higher than similar ones calculated for
other groups but is not extraordinarily high
(70-72].
COMMENTS
It has not been possible to determine the
basal metabolic rate in the inhabitants of
Rongelap. However, the consensus of all phy-
sicians who have examined these people is
that they are not hyperthyroid. The explanation for the large number of subjects with a
high PBI level is, therefore, surely not an
epidemic of hyperthyroidism. An elevation of
thyroxine-binding proteins in serum could,
as in the cases of congenital elevation of thyroxine-binding globulin described by Beier-
waltes and Robbins, cause an increase in serum
PBI without hyperthyroidism [73]. The serum
levels of TBG in the Marshallese measured
by Dr.
J.
Robbins were, however, within
normal limits. The discrepancy between PBI
and BEI, however, suggested the presence of
an iodoprotein in serum. The chromatography of serum iodine showing an iodoprotein
level in the Marshallese of 2.2 ug. per cent
seems to implicate the iodoprotein as associated with the elevated PBI level.
Detailed data are not available on the calorigenic potency of serum iodoproteins but some
results show that most of the iodinated amino
acids in this protein are monoiodotyrosine and
diiodotyrosine [74,75]. These iodoamino acids
are devoid of physiologic activity. Hence an
iodoprotein containing only these iodoamino
acids is likely also to be physiologically inactive.
The reason these persons secrete such an
iodoprotein into the blood is not at all clear.
The data on normal control subjects from the
Eastern United States, who showed 0.8 ug.
VOL. 40, JUNE 1966
885
TABLE tI
KINETIC STUDIES WITH
Gases
An*
:
dat
Theoretical
Uptake
Marshallese |
Normal
0.72
1.0
0.97
2.0
.
42%
33%
wat
No.
Cases
0.08
as
21}
eae
* da is the fraction of extrathyroidal iodide transferred to the thyroid
per day.
:
+ Aa: is the fraction of extrathyroidal iodide excreted in the urine per
day.
toais the value derived by the computer for the fraction of extrathyroidal iodide “seen” by the counter.
per cent iodoprotein in their serum, suggest
that it is a normal albeit minor constituent.
The method of chromatography employed is
such that well under 5 per cent of serum thyroxine (or 0.2 pg. per cent) appears in the
unretarded or iodoprotein fraction. Therefore,
the finding of iodoprotein does not appear to
be a methodologic artifact.
The urinary iodine values were in the normal range. In general it had been expected
that subjects who live close to the sea and
who eat seafood and fish would have a relatively higher iodine intake. The inhabitants
of the Marshail Islands have fish as one of their
main sources of animal protein. Furthermore,
these people are constantly exposed to sea
spray since the highest point on the atoll is
approximately 20 feet above high tide and
the island at its widest is about a quarter of a
mile across.
The data on urinary iodine were used with
the results obtained with I?* studies to calculate the amountof iodine secreted daily by
the thyroid. The value of 76 yg. is somewhat
higher than the figures of 57 wg. per day found
by Stanbury e¢ al. [70] or 58 yg. per day found
by Freinkel and Ingbar [77], but closer to the
value of 70 yg. per day proposed by Riggs
{72]. Unfortunately, nothing is known about
the rate of turnover of the serum iodoprotein.
If it has roughly the same rate of degradation
and the same volume of distribution as thyroxine, we would expect the thyroidal secretion of organic iodine in the Marshallese to be
proportional to the level of organic iodine in
their serum. Adding iodothyronine values to
iodoprotein levels for both Marshallese and
Americans, and multiplying the ratio by the
best value for iodine secreted by normal Americans we obtain:
2.22 + 4.53
T 59 = 86 ue.
da
0.80 13.76 *
HG. per Cay
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