Effect of Acclimation and Heat Stress on Thyroid Hormone Concentration1

J. D. MAY, J. W. DEATON, F. N. REECE, and S. L. BRANTON

USDA, ARS South Central Poultry Research Laboratory, Mississippi State, Mississippi 39762

(Received for publication September 16, 1985)

ABSTRACT In three trials, broilers were exposed to either moderate-constant or high-cyclic tem­peratures for 3 days. Broilers in both treatments were exposed to 41 C and high humidity over a 4- to 6-hr period on the 4th day. Blood samples, collected before and at the end of the heat expo­sure, were assayed for thyroid hormones. Neither acclimation nor severe heat exposure consistently affected triiodothyronine (T3) or thyroxine (T4) concentration. It appears the mechanism of short-term acclimation involves endocrine or physiological responses other than changes in circu­lating thyroid hormone concentrations. (Key words: thyroxine, triiodothyronine, environment)

1986 Poultry Science 65:1211-1213

INTRODUCTION

The basis for short-term acclimation to heat exposure is not clearly understood. Reece et al. (1972) reported 3 days of exposure to high-cyclic temperatures gave broilers significant protection against heat prostration. Recent results suggest thyroid hormone metabolism may be an important factor in response to heat stress. Exogenous thyroid hormones shorten survival time during heat stress (Fox, 1980; May, 1982; Bowen et al., 1984) and reducing thyroid activity lengthens survival time (Fox, 1980; Bowen et al, 1984). Rudas and Pethes (1984) reported thyroxine (T4) concentration was reduced after exposure to 35 C for 1 hr, but triiodothyronine (T3) concentration was not changed. They concluded conversion of T4 to T3 played a major role in the early phase of temperature acclimation. Bowen and Washburn (1985) reported increased T3 and T4 concen­trations in one experiment due to heat stress, whereas neither T3 nor T 4 was significantly increased by heat stress in another experiment.

The objectives of these trials were to further investigate the effect of severe heat stress on thyroid hormone concentrations and determine if short-term acclimation causes changes in hormone concentrations before or during heat stress.

1 Trade names in this article are used solely to pro­vide specific information. This does not constitute a guarantee or warranty by USDA and does not signify that the product is approved to the exclusion of other comparable products.

MATERIALS AND METHODS

Broiler chicks were obtained from com­mercial hatcheries and housed on litter in three trials. In Trials 1 and 2, chicks were a feather-sex cross, with equal numbers of males and females in Trial 1 and males only in Trial 2. Trial-3 chicks were males from Arbor Acre female-line parent stock. Conventional corn-soybean meal diets and water were provided ad libitum. Chicks were warm-room brooded, and temperatures were maintained at 29, 27, and 24 C for Weeks 1 to 3, respectively. Chicks were maintained in the rearing house until 41 days in Trial 1, 49 days in Trial 2, and 22 days in Trial 3, after which chicks were moved to two environmental chambers described by Reece and Deaton (1969). The environmental chambers were maintained at 21 C with 10-C dewpoint, and each chamber was stocked with 40 broilers.

Broilers in one chamber were acclimated to heat by 3 days of exposure to a 24-35-24-C linear cycle with 10-C dewpoint. The minimum temperature (24 C) was at 0200 hr, and the maximum temperature (35 C) was at 1400 hr. At 0900 hr on Day 4, the constant-temperature chamber was set at the same conditions as the acclimatizing chamber, and identical conditions were kept in both chambers througout the heat stress. The temperature was gradually increased in both chambers to reach 41 C at 1400 hr in Trials 1 and 2 and at 1300 hr in Trial 3. At 1000 hr, the dewpoint was increased to 21 C in Trials 1 and 2 and 24 C in Trial 3. Twenty broilers in each chamber were bled prior to stress and 20 were bled during the stress; no broiler was bled more than once.

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1212 MAY ET AL

TABLE 1. Effect of acclimation and heat stress on thyroid hormone concentration

Trial Treatment Time T , 1

Control

Acclimated

Control

Acclimated

Control

Acclimated

0800 1315 0800 1315

0830 1415 0830 1415

0800 1200 0800 1200

3.69a

3.59a

3.67a

3.85a

3.94c

3.29a

3.7lbc 3.45 a b

3.36a

3.48a

3.37a

3.44a

in; 16.2a

17.6a

18.9a

22.5 b

14.4a

14.7a

12.6a

12.9a

24.2a

25 . l a

26.9a

26.6a

a,b,c Within trial and hormone, values without a common letter are significantly different (P<.05). Each value is the mean of 18 or more samples.

1 T3 = Triiodothyronine, T4 = thyroxine.

Blood samples were collected at 0800 and 1315 hr in Trial 1, 0800 and 1415 hr in Trial 2, and 0800 and 1200 hr in Trial 3, and plasma was stored at —20 C until assay. On the day of heat stress, broilers were 57, 56, and 59 days old in Trials 1 to 3, respectively.

Hormone concentrations were determined by double-antibody radioimmunoassay. The T4

antibody, carrier rabbit gamma globulin, and precipitating antibody were purchased from Antibodies, Inc. The T 3 procedure and primary antibody described by May (1978) were used, and the T4 assay was by the same procedure. Radioisotopes were purchased from Amersham Corporation. All samples for each trial were assayed as a single group. The crossreactivity of T3 with the T4 antibody is described by the supplier as .35%. Pooled plasma standards were included in each run, and each lot of T4 anti­body was tested against plasma samples spiked with T 3 . No cross reactivity was detected at the T 3 concentrations in these samples.

Data were analyzed by analysis of variance with mean separation by Duncan's multiple range test as described by Steel and Torrie (1960).

RESULTS AND DISCUSSION

The effects of heat stress and acclimation on T3 and T4 concentrations are presented in Table 1. A comparison of values for acclimated vs. control broilers showed significant elevation of T4 for acclimated broilers during heat stress

in Trial 1. No other significant effect of acclima­tion was observed either prior or during heat stress from T 3 or T 4 . In Trial 3, the heat stress was maintained until 50% of the control broilers died. At that time only 5% of the acclimated broilers had died. This again confirms the report by Reece et al. (1972). The hormone data suggest acclimation does not change circulating levels of thyroid hormones. Any involvement of thyroid hormone metabolism in acclimation must be within the tissues.

Heat stress caused a significant reduction in T 3 concentration in control broilers of Trial 2 and increased T4 concentration of acclimated broilers as noted previously. No consistent pattern was observed. The results for T 4 are similar to those of Bowen and Washburn (1985) but fail to confirm the report by Rudas and Pethes (1984). It appears that the complex physiological response to heat stress does not consistently affect circulating thyroid hormone concentration.

REFERENCES Bowen, S. J., and K. W. Washburn, 1985. Thyroid and

adrenal response to heat stress in chickens and quail differing in heat tolerance. Poultry Sci. 64:149-154.

Bowen, S. J., K. W. Washburn, and T. M. Huston, 1984. Involvement of the thyroid gland in the response of young chickens to heat stress. Poultry Sci. 63:66-69.

Fox, T. W., 1980. The effects of thiouracil and thy­roxine on resistance to heat stress. Poultry Sci.

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RESEARCH NOTE 1213

59:2391-2396. May, J. D., 1978. Effect of fasting on T3 and T„

concentrations in chicken serum. Gen. Comp. Endocrinol. 34:323-327.

May, J. D., 1982. Effect of dietary thyroid hormone on survival time during heat stress. Poultry Sci. 61:706-709.

Reece, F. N., and J. W. Deaton, 1969. Environmental control for poultry research. Agric. Eng. 50:670-671.

Reece, F. N., J. W. Deaton, and L. F. Kubena, 1972.

Effects of high temperature and humidity on heat prostration of broiler chickens. Poultry Sci. 51:2021-2025.

Rudas, P., and G. Pethes, 1984. The importance of the peripheral thyroid hormone deiodination in adaption to ambient temperature in the chicken (Gallus domesticus). Comp. Biochem. Physiol. 77:567-571.

Steel, R.G.D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. A Biometrical Approach. McGraw-Hill, New York, NY.

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