Neural and endocrine development after chronic tryptophan deficiency in rats: II. Pituitary-thyroid axis

Long-Evans female rats, 21 days of age, were weaned and placed on a control (Purina Rat Chow) diet, on a tryptophan deficient diet (T-) and on a diet complete in quality (Purina Rat Chow) but restricted to the daily amount consumed by the rats on the T- diet (pair-feeding). All animals were maintained on these diets for 1 and 2 months and then one group of T- rats was returned to the complete (Purina) diet and kept on this diet for several periods of time (up to 2 months). Growth was interrupted during the period of tryptophan deficiency and pair-feeding, but was restored to normal when the animals were returned to the Purina diet. In control rats, blood levels of TSH, T4 and T3 as determined by radio-immunoassay showed characteristic development patterns. In T- rats, hormonal developmental patterns were similar to those of controls after 1 month on the T- diet but levels fell significantly below control values after 2 months. On the other hand, hormonal levels of pair-fed rats were already significantly low 1 month after treatment and continued to remain low after 2 months. The decrease in thyroid function reported in these experiments as a result of severe dietary restrictions not only may explain the retardation of growth and development characteristic of nutritionally deficient animals, but also suggest some long-term interaction of nutrition and thyroid function on the aging process.