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The effect of thiouracil fed as .1 percent of the ration on economy of gains and carcass quality was studied with Chester White, Yorkshire and cross-bred market pigs.Compared with the controls, five Chester White barrows averaging 162 pounds and fed ad libitum for 41 days on a ration containing thiouracil made .16 pound less daily gain, but required 13.8 percent less feed per unit of gain. Five Yorkshire pigs given thiouracil for the same period averaged .09 pound less daily gain and required 18.8 percent less feed per unit of gain. Nine crossbred pigs fed thiouracil at a different season made .35 pound less daily gain and required 8.4 percent less feed per unit of gain than their controls.All of the pigs in the experimental groups became lazy toward the end of the second week and became increasingly sluggish as the trial progressed. The greater increase in the economy of gain in the Yorkshires as compared to the Chester Whites is believed to be due to a higher natural thyroid secretion rate in the former and consequently a greater reduction in rate of metabolism under the influence of thiouracil. There were no significant differences in the carcasses attributable to the thiouracil treatment.

A Considerable amount of research has been reported on some measures of thyroid development in the bovine fetus, as reviewed by Kossila (1967). Most of this research has been concerned with such aspects as thyroid growth and histological development, the appearance of iodinated components in the thyroid, and thyroidal uptake of radioactive iodine in the fetus and pregnant cow.Lewis and Ralston (1953) reported that calves less than 48 hr. old had higher plasma protein-bound iodine than older animals. Nathanielsz (1969) stated that plasma thyroxine (T4) of the calf is elevated in the first 3 hr. of extrauterine life. This is followed by a rapid fall in T4level. Similar results were noted by Nathanielsz (1969) in newborn lambs. No information has been found on comparative levels of serum thyroxine (T4) together with the capacity of the specific carrier protein, thyroxine-binding globulin, to bind T4in the pregnant cow and the fetal and neonatal calf.

A technique was developed for the determination of body volumes of living animals by air displacement. Two experiments were reported in which body volumes were determined at two levels of negative pressure. In the first experiment, specific gravity of 23 adult female guinea pigs, calculated from body volumes determined at a total vacuum of 320 mm. of Hg., were correlated with carcass fat, water, protein and ash. The correlation coefficients were −.70, 0.67, 0.68 and 0.58, respectively. In the second experiment, body volumes of 25 female guinea pigs were determined at a total vacuum of 120 mm. of Hg., employing a constant volume method of manometeric measurement. Correlation coefficients for air specific gravities thus obtained and carcass fat, water, protein and ash were −.82, 0.81, 0.72 and 0.72, respectively. Other factors which influenced the accuracy of the body volume determinations were changes in chamber temperature and humidity. Variations due to the contents of the digestive tract and composition of the viscera did not significantly alter the relationship between air specific gravity and carcass composition.

Rate of gain, efficiency of feed conversion, physiological effects and certain carcass measures are reported for three lots of steers treated as follows: Lot I, control; Lot II, fed diethylstilbestrol (10 mg. per head daily); Lot III, implanted with combination progesterone-estradiol pellets (1.5 gin. progesterone and 50 mg. estradiol).Implanted steers had a significantly greater daily rate of gain than either stilbestrol-fed cattle or controls. Stilbestrol-fed steers had a significantly higher daily rate of gain than the controls. Average daily gains were 2.30, 2.64, and 3.03 lb., respectively, for the control, stilbestrol-fed and implanted steers. Implanted cattle ate an average of 16.0 lb. of corn per day, stilbestrol-fed steers 15.3 lb. and control steers 14.8 lb., indicating an increased appetite from hormone stimulation.There were no significant differences in dressing percentage, cooler shrink, heart, liver, spleen or hide. There were no significant differences in carcass grade. As estimated by the 9-10-11 rib cut (Hankins, 1946), carcasses from implanted steers had a significantly greater proportion of separable lean, and significantly lower proportion of separable fat than the carcasses for either stilbestrol-fed or control groups.Moisture of external fat, internal fat, Longissimus dorsi, and combined separable fat and lean was not significantly different between groups. No significant differences were found in percentage of ether extract of Longissimus dorsi, or of combined separable fat and lean.There was no difference in cooking shrink or in Warner-Bratzler shear readings.Adrenal glands appeared normal in all groups. Hormone treatment either by implantation or feeding resulted in an increased diameter of the prostatic portion of the urogenital tract and an increase in size of the bulbourethral glands and seminal vesicles, indicating physiological activity due to both hormone treatments. There was some evidence of secretory activity in the prostrate and bulbourethral glands of treated steers. There was no evidence of desquamation or keratinization of epithelium and no evidence of carcinogenesis in any tissues examined.

Shropshire ewes with a high thyroid activity as measured by thyroid output half-time tended to produce faster gaining twin lambs to three weeks of age than ewes with low thyroid activity. It is believed that the faster gains of these young lambs is due largely to increased milk consumption.Daily 1-thyroxine secretion rates per hundred pounds live weight were determined by the extrapolation technique in individual intact Shropshire lambs. The secretion rates by sex were: ewes, 0.08 mg.; rams, 0.07 mg.; and wethers, 0.05 mg. The ewe lambs were secreting significantly more 1-thyroxine than wether lambs (P>.05). The thyroxine secretion of ram lambs was not significantly different from ewe or wether lambs.Ewe lambs with a higher thyroid secretion rate tended to gain more rapidly during a 30-day feeding period. A correlation coefficient of 0.81 was obtained between daily 1-thyroxine secretion rate and 30-day gain which is significant at the 1% level. This relationship is expressed by the regression equation, Ŷ=−1.51+108.46x; where Y=lb. gained and x=mg. thyroxine secreted daily.

In determining the suitability of specific gravity of three untrimmed cuts, i.e., the ham, loin, and shoulder, as measures of carcass leanness, it was found that the specific gravity of each of the cuts was closely associated with specific gravity of the entire carcass. The correlation coefficients were +0.94, +0.96, +0.92 for the ham, loin, and shoulder, respectively, when both cuts from the same carcass were used. However, on a single cut, corresponding values were +0.93, +0.91, and +0.87.The somewhat higher correlations for specific gravity of a single ham to other measures of carcass leanness indicated that the untrimmed ham was a more reliable index of the entire carcass than either a single rough loin or untrimmed shoulder. With the exception of the relationship between specific gravity of the single ham and percent primal cuts on a carcass basis, the correlations for specific gravity of a single ham closely paralleled those for specific gravity of the entire carcass in relation to other carcass measures. Specific gravity of either the entire carcass or a single ham proved to be superior to backfat thickness as a measure of carcass leanness.In examination of correlation coefficients by weight groups, indications were obtained that both backfat thickness and length of carcass were better measures of leanness for lighter weight pigs.

A technique is described for measuring the daily thyroid secretion rate in intact, individual sheep.From 24 experimental ewes divided into four groups and using the extrapolation technique, the following results were observed.The season of the year has a great effect upon thyroid secretion in sheep. In July a daily thyroxine secretion of 0.04 mg. was significantly lower than at any other month readings were taken. The next lowest secretion was observed in September (0.14 mg.). There was a significant difference (P less than .05) between September and March (0.20 mg.).Pregnancy in sheep does not cause a significant difference in thyroid secretion.Lactation creates increased demands upon the thyroid gland as lactating 2-year old ewes had a higher thyroid output than either bred or open 2-year old ewes.Four-year old ewes secreted significantly less thyroxine daily than 2-year old ewes during January, March, May and December. However, differences during July and September were not significantly different.Significant breed differences in thyroid secretion rate between Hampshire and Shropshire ewes were observed only in January and December.

1) The excretion pattern of radioactive substance following parenteral administration of 0.25 μg of Cobalt60-labelled Vit. B12to the young rat has been studied. 2) Urinary excretion of radioactivity reaches a maximum during the first 8 hours after labelled Vit. B12administration, then rapidly declines; fecal excretion rises progressively through the 3rd day. Moreover, administration of a large dose of unlabelled Vit. B12subsequent to radioactive B12significantly increases urinary but not fecal excretion of radioactivity. From this differential excretion pattern, it is inferred that labelled vitamin is altered prior to its excretion into the intestinal tract. 3) The fecal route is the major excretory pathway for radioactivity resultant from parenteral introduction of Co60-labelled Vit. B12; after the 1st 8 hours, the ratio of radioactivity excreted for a 10-day period viafecal and urinary pathway is 7:1, respectively. 4) Sex does not appear to influence excretion of Co60-tagged products. Similarly, administration of large quantities of testosterone propionate to either females or males does not alter the excretion pattern.

No assayable quantities of estrogen from an endogenous source were found in 100-ml. samples of ewes' blood. When a microcrystalline suspension of estradiol was injected intravenously in relatively large quantities, none was found in the blood. It was suggested that the microcrystals had been phagocytized and thus removed from the circulation within 5 minutes or less. The sodium salt of estradiol, when injected intravenously, was readily extractable from 100-ml. samples of ewes' blood if it were first partially hydrolyzed in an NaOH solution. This indicated that a large percentage of the estradiol was bound very rapidly to the blood proteins. Both the sodium salt of estradiol and the microcrystalline suspension when added intravenously, had similar excretion patterns, with the microcrystalline suspension appearing in the urine at twice the excretion rate of the sodium salt of estradiol and remaining at higher rates throughout the period of collection.

A single injection of progesterone in macrocrystalline suspension inhibited estrus and follicular growth in the cycling ewe for a period of 12 to 16 days. The duration of inhibition depended upon the stage of the breeding season when the treatment was administered. In the anestrous ewe, the optimum interval necessary between the injection of progesterone suspension and PMS to induce follicular growth, ovulation, and estrus was 16 days. Approximately 50% of the anestrual ewes treated with progesterone-PMS came into estrus and 37.5% of these lambed following service at the induced estrus. Eighty-seven percent of the anestrous ewes administered 200 µg. of macrocrystalline estradiol 30 hours after the PMS injection exhibited estrus, but only 28.5% of the ewes serviced produced lambs. Although 50 to 200 µg. of the sodium salt of estradiol administered 30 hours after PMS did not influence the incidence of estrus, it definitely enhanced fertility. Five of six ewes serviced following progesterone-PMS-sodium salt of estradiol treatment produced lambs. Ewes that lambed during October were successfully bred 40 to 60 days later while still milking.

The substitution technique for measuring thyroid secretion rate in individual intact animals was adapted to the dairy goat. Normal goats were found to have a sufficiently rapid thyroidal I131output rate to permit application of the method without thiouracil administration. Thiouracil-treated goats had a more rapid I131output rate, but their thyroid secretion rate as measured by the thyroxine substitution method was not significantly different from values found concurrently in normal goats.In both young (2-year-old) and aged (4-to-6-year-old) goats, the thyroid secretion rate was high in the fall, spring, and winter, but declined to about one-half the winter level during July.Comparisons between 8-month-old, 2-year-old, and 4- to 6-year-old animals showed a progressive decline in thyroid secretion rate with increasing age.The percent uptake of injected I131by the thyroid showed no consistent correlation with thyroid secretion rate. However, there was a definite increase in uptake during the summer.Thyroidal I131output half-time (t ½) values were smaller in both young and aged groups during February and May than in July, August, and October. Again there was no consistent correlation with thyroid secretion rate.On the basis of these comparisons, it was concluded that neither thyroidal I131uptake nor output rate are reliable indices of thyroid function in goats exposed to normal environmental variations.Both aged and young lactating goats had a lower thyroid secretion rate than nonlactating controls of similar age during each of the three months in which comparisons were made. Thyroidal uptake of I131was only about one-half as great in lactating as in nonlactating animals. Within lactating groups, there was a negative correlation between milk yield and thyroid secretion rate and also between milk yield and I131uptake. Determinations of I131in milk and blood serum from five animals showed an average maximal concentration of I131in milk that was six times as high as that in serum. During the first 50 hours after I131administration, an average of 26.7% of the dose was secreted in milk and 9.6% retained in the thyroid. These data were interpreted to indicate that through the udder-kidney-thyroid competition for available iodine, a functional iodine deficiency was created, and this resulted in a reduced thyroid secretion rate.

The effects of environmental temperature and thiouracil feeding upon rate and efficiency of gains, blood lipid levels, the weight of some endocrine glands and body organs and certain carcass characteristics of growing-fattening pigs were investigated. A total of four trials were conducted using 54 Chester White and Yorkshire pigs at an average initial weight of approximately 70 kg. They were slaughtered at an average weight of about 90 kg. The environmental temperatures were either 4° C. (cold) or 27° C. (warm), and the rations were identical except that one group was fed 0.15% thiouracil.The feeding of thiouracil beyond 30 days caused a marked decrease in activity, and after 40 days there was a marked decline in appetite. The thiouracil-fed pigs lost their bloom and developed a mottled appearance in some areas of the skin, similar to one of the symptoms of Addison's disease in man. The thiouracil-fed pigs made slower gains and had higher blood lipid levels, a lower 24-hr, off-feed shrink, shorter carcasses, heavier thyroid glands and livers than control pigs. The cold environmental temperature increased blood lipid levels, resulted in heavier thyroid glands and liver weights, but decreased feed efficiency, average backfat thickness and percent of fat trim. Although data on the average height of the thyroid acinar cells were obtained for control pigs only, height was increased significantly by the cold environments, indicating greater thyroid activity at the low temperature.The Yorkshire pigs had higher blood lipid levels, less backfat, a lower percent of fat trim, a greater percent of lean and primal cuts, longer carcasses and a higher specific gravity of the ham than the Chester White pigs.

The conventional apparatus used earlier in this laboratory (Gnaedinger et al., 1963b; Hix et al., 1964) for measuring body volume was modified by the addition of two chambers, identical to those in use. The addition of these chambers resulted in the formation of a suppressed zero measuring system in which the relatively small difference in volume between the subject and an inert reference was determined, rather than the total volume of the subject.The specific gravity of 24 hogs was determined and the values were compared with chemical analysis data. Specific gravity values determined by helium dilution were significantly correlated (r=0.59) with those obtained by air displacement. However, specific gravity values determined by both methods were not significantly correlated with chemical composition, except for percent ash. The major source of error involved in both the air displacement and helium dilution procedures was due to differences in relative humidity between the subject and reference chambers and to changes in relative humidity within the subject chamber.Chemical analysis showed the average composition of the 24 hogs to be 52.00% moisture, 30.36% ether extract, 15.21% protein and 2.57% ash. A highly significant correlation of −.98 was obtained between percent total moisture and percent total ether extract and a regression equation was computed for predicting total fat from total body water. Of the various body compartments, the chemical composition of the shoulder and ham was most closely related to composition of the whole animal. Equations were computed for predicting the chemical composition of the whole animal from the percent composition of the shoulder. However, the standard errors of the estimate showed that this prediction would not be too useful.

Twenty-four pigs weighing 181 to 220 lb. each were dissected into seven separate body compartments (carcass, head, pluck, G.I. tract, G.I. contents, blood and hair). Each compartment was frozen, sawed into thin strips (except for the hair), ground until homogenous and sampled for chemical analysis. The samples were then analyzed for fat, moisture, ash and protein. The composition and contribution of each compartment to the whole animals are presented. Results indicated that fat content was the most variable component. The composition of the empty bodies (whole animal minus G.I. contents) on a fat-free basis was extremely constant and was nearly identical to the composition of the fat-free carcasses. Equations for estimating the fat content of the whole and empty body from the water content were computed.A highly significant correlation (r=−.58) was obtained between the water content of the fat-free body and total body fat, indicating that the water content was influenced by the total fat content of the body. Thus, results indicated that the pigs used in this study were not chemically mature, and that estimation of fat from a constant value for the moisture content of the fat-free body was not valid.Of the various body compartments, the carcass made the greatest contribution to the composition of the whole body. It accounted for approximately 74% of the water, 90% of the fat, 76% of the protein and 79% of the ash. The errors that would be involved by failing to analyze the various body compartments are presented and can be used as guides to attain the desired accuracy in body composition studies.

Four trials were conducted with pigs to ascertain the effects of temperature, controlled feeding and fasting upon plasma lipid levels. An apparent diurnal variation indicated that maximum lipid levels occurred in the early morning hours. However, this was later shown to be due to temperature, with lower readings resulting in elevated lipid levels, whereas, higher temperatures resulted in lower plasma lipid levels. There was no significant difference in plasma lipid levels between fasted pigs and those given a constant feed intake. However, the lipid levels for the fasted pigs were higher than for fed pigs at the termination of both trials. Within 8 hours after allowing normal feed intake following a 48-hour fasting period, there was a significant reduction in blood lipid levels. Results indicate the importance of controlled environmental conditions in studies involving blood plasma lipid measurements.

Carcass comparisons of the boars and barrows of different castration weights showed that the boars and 180 pound castrates had a higher percent of lean in the rough loin, less backfat, longer body length and a higher liveweight and carcass preferred cut yield. Quality of the pork produced by boars was inferior in all respects. Acceptability tests showed that the boars produced pork that had a definite “off” flavor and odor. The accessory sex glands and kidney weights of the boars were heavier and the weights decreased very rapidly as the weights of castration decreased. There was no significant difference in sex accessory gland and kidney weights between normal barrows and barrows implanted with 193 mg. pellets of testosterone proprionate. No objectionable odor or flavor was found in pork produced by animals castrated an average of 5 weeks prior to slaughter. Rate of gain and amount of gain per pound of feed was not affected by testosterone pellet implantation, delayed castration or by non-castration.

Body temperature development of baby pigs was studied under various conditions of environmental temperature. The effects of chilling on body temperature and blood changes in pigs ranging in age from birth to three weeks old were studied.The body temperature regulating mechanism in the new-born pig is not fully developed. There was a body temperature droy of 3° to 13° F. in baby pigs during the first 30 minutes after birth, the amount of drop being related to the size of the pig and the environmental temperature. The initial drop was followed by a gradual return toward normal, which in environments of 60°–75° F. was reached in about two days, and in environments approaching freezing temperatures was reached in about ten days.Pigs 24 hours old withstood a temperature of 34° F. with less drop in body temperature than pigs under 4 hours old.There was a significant correlation between the weight of a pig and its ability to adapt itself to its environment. Small pigs in a fasting state passed into a coma sooner when chilled than pigs of comparable size having access to sow's milk.Chilling of two-day-old pigs caused the blood hematocrit to decrease, which is a reversed reaction for the normally functioning temperature regulating mechanism.There was an increase in blood sugar when one-day-old pigs were chilled at 34° F. Pigs fasted eight hours before chilling showed less glucose increase than those fasted three and five hours.

The literature on the physiologic effects of thiouracil and thyroidal substances has been reviewed, with particular attention to possible applications of these compounds in the principal classes of livestock, namely, dairy cattle, beef cattle, sheep, swine, and poultry.By virtue of its thyroid-inhibiting property thiouracil will cause a depression of milk and egg production and when given to very young animals will cause growth stasis and varying degrees of cretinism. When given only during the last three to four weeks of the fattening period in chickens and turkeys, however, and the last four to six weeks in swine, this drug has caused significant improvement in the economy of gains in most cases with little depression in the rate of gain. Increased fat deposition has usually been observed in poultry, but not in swine. Results with thiouracil in beef cattle and sheep have been inconsistent, presumably because the optimal conditions for administration of the drug have not been established.The use of thyroidal substances for the stimulation of milk production has been investigated quite extensively. Within a limited range the percentage increase in milk yield is roughly proportional to dosage. Fat percentage also increases, the increase usually appearing at lower dosage levels than the rise in milk yield. With the exception of a small and variable decrease in milk nitrogen no other important alterations in milk composition have been reported. Thyroid hormone is not transmitted to the milk. It is concluded that milk from cows fed iodinated protein is safe and nutritionally adequate.It is well established that thyroidal stimulation of dairy cows must be accompanied by an increase in nutrient intake if losses in body weight are to be avoided. If sufficient extra nutrients are supplied to support both the increased metabolism and the increase in production, normal body weights are maintained. It is extremely important to keep the dosage within the “physiological tolerance range” of the cow, particularly in long time feeding programs, in order to avoid overdosage. On the basis of present information it appears that thyroidally active iodinated protein could be used profitably during periods of milk scarcity to stimulate production for periods of about two months. Under good conditions of feeding and management and proper regulation of dosage it could be used in commercial herds for more extended periods. Until more information is available on the long term effects, the use of iodinated protein in purebred breeding herds would appear questionable. Iodinated protein is poorly tolerated during hot weather. However, further research at low levels of dosage would be of interest.Iodinated protein has proved beneficial in improving libido and fertility in sluggish and inactive bulls, and may provide a means of extending the period of usefulness of some herd sires. It has also been found to overcome “summer sterility” in rams although no practical experiments on the use of this method for lengthening the breeding season of sheep have been reported.Conclusive information on the effects of iodinated protein on lactation in sows has not thus far been reported. When fed to young growing pigs, within a critical dosage range, iodinated protein has caused some increase in the growth rate together with a slight improvement in economy of gains. The pigs appear thicker bodied, more mature and more uniform in conformation than control animals. Results on carcasses range from no effect to some increase in weight of loin and bacon cuts.In chicks iodinated protein consistently causes a pronounced acceleration of feather growth. Effects on body growth are variable, depending upon the dosage level given, the breed and strain of chicks and probably other factors.Small amounts of iodinated protein given continuously in the ration have been reported to prevent partially the summer decline in egg production and also to retard the yearly rate of decrease in egg production with advancing age. Since agreement among several laboratories is poor, however, more research will be required to assess the possible role of thyroidal stimulation in egg production. Iodinated protein has also been reported to improve egg shell quality, but again there is some variability between laboratories, indicating the need for further research.Because of the newness of this line of investigation any conclusions that can be drawn at this time as to the potential usefulness of alterations of thyroid function in the various fields of animal production are definitely limited in their scope. As in all new fields of investigation the solution of one problem invariably calls attention to several others that are awaiting solution. The introduction of goitrogens and thyroactive iodinated proteins, which permit control of the thyroid function at will, not only gives promise of some applications of economic importance, but also provides a research tool that should help to advance a step farther the frontiers of our knowledge of the physiologic processes upon which livestock production is based.

The absorption and elimination of thiouracil was studied in calves and goats following oral and subcutaneous administrations.Maximum blood plasma thiouracil concentrations were reached in 4 to 8 hours following oral administration and in 2 to 4 hours following subcutaneous administration. Over the range studied, blood plasma thiouracil concentrations were approximately proportional to dosage levels.Single doses administered subcutaneously gave approximately twice the blood plasma thiouracil concentration as the same dosage level administered orally.Essentially uniform blood plasma thiouracil concentrations were maintained in ruminants with oral and subcutaneous administration at 12 hour intervals.Thiouracil was practically eliminated from the blood stream within 24 hours after cessation of administration.

The relationship between the thyroid, ovary and adrenal glands is very complicated. The activity of the adrenal cortex undergoes cyclic variation in female animals, with a maximal activity during the oestrous phase (Anderson & Kennedy, 1932; Foster, 1934, and Zondek & Burstein, 1952). The thyroid gland also undergoes cyclic variations, its activity being greatest during oestrus (Soliman & Reineke, 1954, and Soliman & Badawi, 1956). Oestrogen seems to be the common factor, as its administration to animals stimulates the functions of both the thyroid and the adrenal glands (Soliman & Reineke, 1955, and Tepperman et al., 1943).It is also evident that hyperthyroidism stimulates adrenal function as reported by Wallach & Reineke(1949), while adrenocortical extracts markedly diminish thyroid function (Soffer et al., 1949). With these findings in mind, it was decided to investigate how the adrenal gland affects the response of the thyroid to oestrogen.

Cellulose acetate or disk electrophoresis of human semen and seminal plasma revealed between seven and ten zones of migration, depending on the technique used. Disk electrophoresis of washed human spermatozoa resulted in a pattern of protein separation that could not be distinguished from those obtained by disk electrophoresis of semen or seminal plasma. This is evidence that proteins being removed from spermatozoa are very similar to those present in seminal plasma. Immunological properties of human semen were investigated by immunological double diffusion and immuno-electrophoresis. The latter technique demonstrated at least six antigenic components in semen. These could be resolved into four antigens common with serum and four antigens specific to semen. Immuno-electrophoresis resolved at least ten semen components. These were subdivided into five common to serum and five specific to semen. Finally, there is limited evidence to indicate a substance common between semen and saliva but absent in serum.

Thyroid secretion rate (TSR) was determined in lactating and non-lactating rats by the thyroxine substitution and `direct output' methods. In lactating rats, apparent TSR as determined by the substitution method was significantly greater than that of controls when dietary iodine was deficient. By the direct output method, TSR was significantly reduced even when dietary iodine was adequate. It is concluded that the substitution method measures the physiological demand for thyroid hormone, which is greater during lactation, but represents true TSR only if iodine supplies are adequate. Results obtained by the direct output method show the actual TSR to be significantly reduced both during lactation and when dietary iodine is limited. Mean thyroid acinar cell height significantly increased as thyroidal iodine was depleted. It is concluded that, during lactation, TSH was elevated but iodine was insufficient to support the demand for thyroid hormone. The low serum T4level observed in lactating rats, as determined by a competitive binding method, supports these conclusions.Analysis of the methods of computation of TSR by direct output showed that (1) as thyroidal iodine supplies were depleted, thyroidal iodide accumulation became more efficient, (2) the iodine-deficient thyroid recycled a greater percentage of the available iodine, (3) thyroidal 131I uptake was reduced in lactating rats, (4) total thyroidal iodine was inversely correlated to its release rate (K4) from the thyroid, and (5) thyroidal iodine, and not K4, was the major factor influencing TSR under these experimental conditions.