The way in which environmental and physiological factors regulate the
plasma concentrations of thyroid hormones in birds has been investigated.
The administration of goitrogens in the diets of growing chickens
was found to depress the levels of plasma T„ and T, and increase the
activity of the thyroid follicular cells.
Daily rhythms in the levels of plasma T₃ and T₄ were found to be
inversely related and independent of the circadian system and the pineal
gland. The concentration of plasma T₃„ increased from its lowest levels at
the onset of a light period and was associated with an increase in heart
rate and a decrease in the concentration of plasma T₄ . Fasting abolished
the daily rhythms in plasma thyroid hormones and was associated with a
decrease in the concentration of plasma T₃ and an increase in the concentration of plasma T₄, while feeding, regardless of the time of day and
photoperiod, resulted in an increase in the concentration of plasma T₃ and
a decline in plasma T₄.
Ducks transferred to a long day photoperiod showed an increase in the
concentration of plasma T₄ during the refractory period of the breeding
cycle which coincided with the post-nuptial moult. The initiation of
photorefractoriness and moult in the W.illow Ptarmigan were not associated
with any marked fluctuations in plasma T₃ and T₄, In quail the concentration
of plasma T₄ responded to changes in daylength while the levels of both T₃
and T₄ were affected by the reproductive state of the bird.
The concentration of pituitary TSH in ducks was affected by changes
in thyroid status: goitrogens increased whereas injections of T₄ decreased
levels of TSH as measured by immunocytochemistry and bioassay. Injection
of TSH into laying hens was shown to increase the levels of both plasma T₃
Elevated levels of plasma thyroid hormones were found during the first
weeks of life, which declined as the birds became sexually mature. The
concentration of plasma T₃ increased in broody bantam hens within one or
two days after hatch and rose further after the chicks hatched. An
injection of prolactin into hens increased the concentration of plasma T₃.
The stress of repeatedly handling chickens and turkeys resulted in a
decrease in the levels of plasma T₄ whereas temperature and dehydration
stress affect the concentrations of T₄. and T₃ principally by a change in
A reduction in the levels of plasma induced by thyroidectomy,
fasting or by an increase in environmental temperature was associated
with a decrease in the rate of heat production. An injection of into
thyroidectomised birds resulted in a sustained increase in the levels of
The concentration of plasma triglycerides were found to be positively
correlated with the levels of plasma T₃. Plasma lipoproteins were demonstrated to bind both T₃ and T₄.
Plasma levels of T₄ were not useful for prediction of subsequent egg
production in pedigreed hens and neither were the concentrations of plasma
T₃„ and T₄ related to shell quality as measured by specific gravity and gas
conductance. A relationship was established between the levels of plasma
T₃ and improper healing of the navel in selected lines of newly hatched