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Effect of a Tryptophan Deficiency on Thyroid Gland, Growth Hormone and Testicular Functions in Chickens^1,2,

Bioresearch Laboratory, Department of Animal Sciences, University of Vermont, South Burlington, VT 05401 and ^* Department of Animal Sciences, Rutgers — The State University, New Brunswick, NJ 08903

Broiler chicks from 2 to 4 weeks of age were fed control (0.23%) and deficient (0.115, 0.058%) levels of L-tryptophan. Separate groups of control chicks were pair-fed daily with the deficient chicks. Deficient chicks grew less efficiently than did pair-fed controls. Plasma triiodothyronine (T₃) was elevated in deficient chicks, especially compared to pair-fed controls, and this was accompanied by lower reverse T₃. Plasma thyroxine (T₄) was also reduced with 0.058% tryptophan. However, thyroid weights and follicle diameters were unchanged. Reductions in weight gain and bone growth, increases in pituitary and pineal weights, and elevations in plasma GH occurred in the tryptophan-deficient chicks apart from any effect due to reductions in feed intake. Comb and testis size, and plasma testosterone were little affected by the deficiency. Based on changes in plasma T₃ levels and feed conversion efficiencies, we conclude that tryptophan-deficient chicks show signs of relative hyperthyroidism and energy wastage compared to pair-fed controls. The elevations in plasma growth hormone and T₃ support the concept that serotonin is a negative modulator of the synthesis or release of these hormones in chickens.

KEY WORDS: • tryptophan • thyroid gland • growth hormone • testes • endocrine function

¹ Vermont Agricultural Experiment Station (VTAES) Journal Article 514.

² This study was supported by grants from Agway Inc., Syracuse, NY and the University of Vermont Research Advisory Council, #80-02, and by funds from the VTAES project Hatch 268, New Jersey AES (NJAES) projects #18141 and #18433 (supported by State and Hatch Act Funds) and National Science Foundation Grant PCM 80-22727.

Manuscript received 13 December 1982.