Effect of Dietary Iodine on Thyroxine Secretion Rate of Lactating Cows

E. W. SWANSON
Department of Animal Science
University of Tennessee, Knoxville 37916

Abstract yields increase (13), so there is high probabil-

Supplemental iodine, 10O mg daily, ity that high producing cows excrete more
was added to low iodine diets of s e v e n total iodine in milk than do lower producers.
Holstein cows which were compared to Milk iodine concentration is correlated with
paired controls. Treatment was from 6 to plasma iodide and iodine concentrations in
8 weeks prepartum through 28 weeks of feed and water (1, 6, 9). Flamboe and
lactation. Plasma thyroxine (T4) and T4 Reineke (3) and Lorscheider et al. (10) have
disappearance rates after injection of 20 postulated that with limited iodine intake,
mg T4 were determined four times at 2- lactation may become self limiting due to re-
month intervals. Iodine supplemented dueed thyroid function resulting from a
cows averaged higher than controls in deficiency of iodine caused by excretion in ex-
plasma total iodine, protein-bound iodine, cess of intake. Lactation records of cows on
and T4. Average differences in T4 turn- low iodine, goitrogenie diets were reported by
over rates were not signffieant, but over- Hemken et al. (5) to increase when supple-
all T4 secretion or disappearance rates mental iodine was fed.
were higher for the supplemented cows The objective of my experiment was to com-
in the last two periods. Plasma thyroxine pare effects on lactation and thyroxine secre-
secretion rates of both controls and iodine tion rates of a low iodine basal diet with the
supplemented cows increased slightly same diet supplemented with 100 mg of iodine
with advancing lactation. Incipient iodine (KI) daily. The supplemental iodine was set
deficiency was indicated in the control in excess of expected requirements to assure
cows at the end of the trial by low milk a constant surplus of iodine for thyroid activi-
iodine and blood iodide concentrations. ty. Results of this experiment should test the
Milk yields of control cows averaged 733 hypothesis that lactation may become self-
kg higher than yields of supplemented limiting because of the possible effect of iodine
cows in 28 weeks. There was no indica- depletion on thyroxine secretion.
tion that the iodine deficient diet caused
hypothyroidism in high producing cows. Experimenta! Methods
A group of 14 Holstein cows was selected
Introduction
from the University of Tennessee herd which
had previous lactation records and had ex-
The mammary gland is an important route pected calving dates within 40 days in October
for excretion of iodine. Jersey cows fed normal and November to minimize any seasonal effect.
rations without iodine supplementation and Cows were paired on previous lactation yield,
milking at the rate of 10.5 kg daily excreted age, and calving date, and one of each pair was
6.8~ of a ~axI dose in milk, 42.8% in urine, randomly assigned to either control or iodine-
and 31.5% in feces (11). The excretion rates supplemented treatments (Table 1).
of continuous daffy doses of ~z~Iby three Jersey Both groups were started on experimental
cows averaging 10.8 kg milk daily over 5 feeding about 8 weeks before expected parturi-
months averaged 10.5% in milk, 48.5% in tion and continued through the 28th week
urine, and 35.6% in feces (13). Other studies of lactation. They were fed corn silage ad
have shown from 3.3 to 16.5% (12), from libitum plus a maximum of 2.5 kg/day of
6.0 to 9.4% (7), fi'om 3.9 to 16.4% (8), and mixed alfalfa-grass hay plus a liberal allowance
from 7.3 to 14.9% (9) of daffy iodine intake of concentrates. The concentrate formula was
excreted in cow's milk. corn, 53; oats, 20; soybean oil meal, 25; plain
Concentration of iodine in cows milk does salt (NaC]), 1; and dicaleium phosphate, 1.
not change (11) or may decrease as milk Concentrates were fed at 5.45 kg daily before
calving. After calving, allowances were in-
creased in ratio to milk yields at about 1:2.5
Received for publication May 11, I972. with a maximum daily feed of 16.3 kg concert-
1763
1764 SWANSON

trates. Feed allowances for paired cows were times the preliminary plasma thyroxine times
alike which resulted in equal average feed in- the calculated thyroxine distribution volume
takes by the two groups. A supplement of 100 at the time the 20 mg dose of thyroxine was
mg of iodine, as KI diluted in cane sugar to injected.
1% iodine, was added once daffy to the feed Samples of feed and milk, taken at about
of each cow in the supplemented group. monthly intervals, and all preliminary plasma
Thyroxine secretion rates (TSR) were deter- samples were analyzed in triplicate for total
mined four times at approximately 2-month iodine by an alkaline fusion method adapted
intervals, beginning 1 month before calving. from and similar to that for plasma protein
The TSR method was based upon the chemical bound iodine (2),
method proposed by Post and Mixner (14). Milk was weighed and recorded at each
After collection of two daffy preliminary blood milking. Composite daily milk samples were
samples, 20 mg of L-thyroxine in alkaline analyzed for fat and total solids at biweekly
aqueous solution were administered in- intervals. In a preliminary test, iodine content
travenously. Blood samples were subsequently of foremilk compared to complete milking was
collected in oxalated tubes at 15, 24, 39, 48, determined with two cows which had been
and 63 hr post injection. Plasma was separated milked for several months without exposure of
and refrigerated at 5 C, until thyroxine deter- udders or milking equipment to iodine com-
minations could be made. Thyroxine was de- pounds. Foremilk from the two cows contained
termined by a competitive protein binding 20 and 29 /zgfliter of iodine, while the total
method utilizing a commercial preparation milk contained, respectively, 20 and 27
(Tetrasorb) according to the manufac~xer's /~g/liter. All milk iodine analyses in this study
instructions, a Thyroxine secretion rate was cal- were on foremffk samples taken after thorough
culated as the daffy fractional decline rate cleansing of udder and teats with a hypo-
chlorite solution. Care was taken to prevent
washing udders or dipping teats of cows on
1 Tetrasorb-125- T-4 Diagnostic Kits were ob- this experiment with iodine solutions.
tained from Abbott Laboratories, North Chicago, Statistical significance of treatment differ-
Illinois 60064 ences was determined from analysis of vari-

TABLE 1. Characteristics of paired experimental cows with preliminary and experimental lactation yields.
Milk yield
Pair Calving Calving Body
& date age weight Previous Experimental lactation
Cow 10 months 10 Months 28 Weeks
(no.) (month) (kg) (kg)
Controls
1-342 10-1 70 707 9,580 10,896 8,043
2-427 10-17 50 630 7,999 10,449 7,640
3-363 10-25 67 670 7,021 10,261 7,702
4-490 11- 1 36 576 5,994 7,831 5,921
5-449 10-7 46 573 5,268 8,088 5,890
6-470 9-26 37 650 5,608 4,820 4,9.79
7-489 11-i 36 523 5,054 4,902 3,878
Avg 10-21 49 618 6,646 8,178 6,193
Iodine
supplemented
1-374 10-8 62 567 8,979 6,446
2-410 11-11 57 744 8,256 8,147 6,9,91
3-388 10-1 60 675 6,954 7,985 5,852
4-475 10-2 37 648 6,011 7,871 5,484
5-404 10-18 58 609 5,485 7,492 5,756
6-438 10-24 48 646 5,963 5,778 4,885
7-479 10-15 37 639 5,269 3,606 3,502
Avg 10-16 51 647 6,702 •• 5,460

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 I O D I N E AND THYROXINE SECRETION 1765

TABLE 2. Average consumption and composition of feeds fed during 28-week lactation comparison~.
Daily Composition-as fed
Feed intake
Water Crude Ether Crude N-free
protein extract fiber extract Ash Iodine
(kg) (~) (~g/kg)
Alfal~a-
grass hay 2.48 13.7 15.8 1.3 25.8 36.7 6.7 102
Corn silage 13.10 71.1 2.7 .7 6.4 17.7 1.4 50
Concentrate 13.44 11.5 19.7 1.2 4.6 58.3 4.7 62
Composition data are averages of four composite samples of each feed obtained at about 2-month
intervals.

ance. Cows were paired according to outcome was therefore about 2 mg per cow daffy.
groups for milk yields; for other comparisons, T h e four tests for TSR were at the periods
pairings were ignored. shown in Table 3. Average plasma iodine and
thyroxine measures for each group at each pe-
Results
riod are in Table 3. Feeding 100 mg supple-
The bases for pairing the 14 cows and their mental iodine daffy resulted in significantly
milk yields in this expermient are in Table 1. greater values for total plasma iodine and
The pairs were well balanced on previous milk plasma protein bound iodine at all periods.
yields and age but differed somewhat in body Differences due to treatment in plasma
size and dates of calving. I n terms of experi- thyroxine, thyroxine disappearance rate, and
mental milk yields, the first five control cows daffy TSR were not statistically significant
produced more than expected from previous (P>.O5) in the first two periods but were sig-
yields and age, but only two cows (404, 475) nificant in the last two periods. Iodine supple-
of the iodine supplemented group exceeded mented cows tended to have slightly higher
their expected yields in the same manner. plasma thyroxine but slower thyroxine disap-
The proximate composition, iodine content, pearance rates. Averages of all calculated
and average actual daffy intakes of feed are in volumes of thyroxine distribution were 65.14
Table 2. Average iodine intakes from the feeds liters for controls and 68.96 liters for I-supple-
were 1.75 mg per cow daily. The water which mented cows -- a difference not statSstically
was from Knoxville City supply contained 2 significant ( P > . 0 5 ) . Estimated TSRs were
/~gI/liter. Total feed and water iodine intake alike in both groups prepartum and then were

TAULE 3. Plasma iodine, thyroxine, and thyroxine turnover and secretion rates of cows on low iodine and
iodine supplemented diets.
Thyroxine
Group Total Plasma
mad plasma protein- Plasma Disappear- Secretion
period I bound I anoe rate rate
(#g/100 ml ) ( g/day ) ( mg/day )
Controls
Prepartum 6 . 5+- .7~ 4.7-+.3 6.1-+.4 34.0-+3.1 1.24__+.07
1-2 Months lact. 11.2-+ 1.4 4.3-+ .4 5.6 ± .6 40.7 -+1.3 1.41 +__.14
3-4 Months lact. 6.0_-..5 4.1-+.3 5.7-+.4 39.6-+ .8 1.48___.16
5-6 Months ]act. 4.1-+ .2 3.5-+.2 5.3-+.3 48.5___3.7 1.68___.11
Avg 7.0-+ .6 4.1 -+.2 5.7-+ .2 40.7-+ 1.9 1.45___.07
Iodine supplemented
Prepartum 38.2-+ 1.2 6.1 -+.3 5.2 -+.2 34.7-+2.9 1.26__..10
1-2 Months fact. 26.1 -+1.8 8.3 -+.7 6.4 -+.3 38.8 -+1.7 1.59 _+.20
3-4 Months laet. 28.2 -+3.2 8.8 ± .7 7.6-+ .6 35.2 _+2.4 1.73 ±.11
5-6 Months laet. 2'4.4+4.0 7.3-+.7 6.4-+.2 39.4-+1.0 1.80±.10
Avg 29.3-+ 1.7 7.6 -+.4 6.4-+ .2 37.0 + 1.7 1.59-+ .08
"Means __. standard errors of mean.
IOURNAL OF DAIRY SCIENCE VOL. 55, NO. 12
1766 SWANSON

TABLE4. Average daily milk yield and milk fat, solids-not-fat, and iodine in five monthly periods, a
Group and Nov. April Overall
constituent Dee. Jan. Feb. Mar. May Average

Controls
Daily yield (kg/day) 35.1 34.4 31.2 28.7 25.8 31.6
Fat test (%) 3.38 3.15 3.37 3.51 3.65 3.41
Solids-not-fat ($[) 8.65 8.69 8.74 8.68 8.66 8.68
Iodine (/,g/liter) 52.8 56.4 16.5 33.5 14.3 35.7
Iodine
supplemented
Daffy yield (kg/day) 33.1 29.9 26.6 24.4 21.4 27.8
Fat test (%) 3.39 3.26 3.50 3.46 3.72 3.47
Solids-not-fat (%) 8.56 8.65 8.69 8.60 8.58 8.62
Iodine (t,g/liter) 325.7 830.1 518.7 580.0 799.3 610.8
a Iodine analyses was from one milk sample eaeh period; solids-not-fat from 2 to 5 samples per peri-
od; fat tests average of 3 to 7 per period. Seven cows in each group.

slightly higher in the iodine supplemented tremely low milk iodine. These observations
cows during lactation. As lactation advanced, indicate that when the supply of iodine is
TSR increased slightly in both groups. limited, the amount lost through the mammary
Averages for milk yields and milk fat, solids- gland can be effectively reduced, and the
not-fat (SNF), and milk iodine concentrations available iodine supply is recycled efficiently
are in Table 4 for five periods during the ex- enough to prevent reduction of thyroxine out-
periment. Iodine supplementation caused no put.
significant difference in fat and SNF tests. The restricted iodine intake of the controls
Milk iodine of the supplemented cows did not cause reduction in milk yields com-
averaged about 17 times that of the controls pared with the iodine supplemented cows (Ta-
and was 55 times as high in the last period. ble 4). Comparing each cow's production dur-
After about 6 months lactation on the low- ing the experiment with her previous 10-month
iodine diet, milk iodine concentration was re- yield (Table 1) showed that in 28 weeks of
duced to 14.3 #g/liter which is typical of the experimental lactation the controls aver-
milk from cows on natural feeds from iodine aged 93.3% of their previous 10-month yield
deficient areas (1, 9). During the first two compared to 82.4% for the supplemented
sampling periods the control cows were cows. All of the control cows completed 10-
averaging about 34 liters milk daily containing month records after the experiment ended, and
55 ~g/liter or 1.87 mg iodine excreted daily these averaged 122.5% of their previous 10-
in their milk. By the last period their average month milk yields. These observations are not
milk yield was 25 liters per day and their daily in agreement with those of Hemken et al. (5),
milk iodine excretion was .35 rag, or about who reported a lactation stimulus from iodine
20% of their feed iodine intake. Correlation supplementation following approximately 1
between TSR and daily milk yield of cows at year on a corn silage-concentrate diet without
the time the three TSR determinations were added iodine.
made during lactation was not statistically sig- Rapid iodine depletion might have been ex-
nificant (r = --.18). pected from the low iodine feeds in the present
Discussion
experiments. Hay and corn in this experiment
had been purchased from areas in Indiana and
Results of this investigation have not sup- Ohio which are generally recognized as iodine
ported Flamboe and Reinekes' postulation (3) deficient. The method of analysis may have
that heavy lactation could cause a reduction underestimated total iodine content of the
in thyroid activity in the cow. From the first
to the fifth month of lactation daily TSR in- feeds. Iodine content of feeds reported by
creased .27 mg for the controls compared to .21 Hemken et al. (6) was approximately 10 times
mg for the iodine supplemented cows. The in- that of feeds in this experiment. Hemken et al.
creased TSR of the controls oeeurred in spite (4) noted that no difference in first laetation
of low plasma iodine which resulted in ex- experimental milk yields had occurred in their
JOURNAL OF DAIRY SCIENCE VOL. 55, N O . 12
 I O D I N E AND THYROXINE SECRETION 1767

cows fed goitrogenic versus nongoitrogenic (6) Hemken, R. W., M. Oskarsson, L. B. Fry-
diets, but it was not established that the cows man, and J. H. Vandersall. 1971. Iodine
were actually de/~cient in iodine. The possibil- content of milk and forage. J. Dairy Sci.,
ity that a period of disturbed iodine metabo- 54:450.
lism extending over 1 year may prevent opti- (7) Lengemann, F. W., and E. W. Swanson.
1957. A study of the secretion of iodine
m u m lactation, as was observed in the second
in milk of dairy cows using daily oral doses
report by Hem.ken et al. (5) has not been dis- of 11~*.J. Dairy ScL, 40:216.
proved by the present experiment. Results of (8) Lengemarm, F. W., E. W. Swanson, and R.
this experiment indicate that ff such a lactation A. Monroe. 1957. Effect of season on secre-
depressing effect should occur, it would not t_ion of iodide in milk. J. Dairy Sci., 40:387.
likely be caused by a lowered TSR, (9) Leskova, R., and M. Weiser. 1969. Iodine
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