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The injection of thyrotropin releasing hormone into cattle resulted in a rapid decrease in the T4/T3 molar ratio. 2 breeds of cattle, Shorthorn and Africander Cross were studied. The decrease in the T4/T3 molar ratio was significantly greater in the Shorthorn breed. It is concluded that acute stimulation of the thyroid gland with TRH results in enhanced release of both T3 and T4 and that T3 is discharged more rapidly than T4.

Plasma total thyroxine (T 4) concentrations were measured by radioimmunoassay throughout one year in 10 sheep maintained indoors under natural light and consuming a constant amount of feed. Plasma T4 concentrations varied throughout the year, the highest and lowest values occurring in the spring and autumn months respectively. The mean of all observations was 5 �4�0�1 ,ug/dl. The changes observed did not appear to correlate with changes in ambient temperature, day length or with reported changes in wool growth.

Plasma concentrations of dexamethasone have been measured in sheep during an 8-day infusion of dexamethasone-21 phosphate. The dexamethasone concentration profiles generally revealed a reproducible pattern with three phases--a peak during the first 48 h infusion which was followed by falling concentrations during the next 5 days, and a small increase in dexamethasone concentration during the final 24 h infusion was not uncommon. The pattern of dexamethasone concentrations was retained when dosage was arranged in such a way as to infuse increasing quantities of hormonal analogue as infusion progressed. Aspects of the metabolism of the analogue are discussed. Endogenous thyroxine and cortisol were significantly depressed during infusion. In these experiments wool was completely shed in three out of four animals dosed at a rate of 8.5 mg dexamethasone/kg0.75. The recovery of wool growth to its pretreatment values occurred by about one month after infusion. The consumption of food and body weight increases were satisfactory during the post-infusion period.

Plasma thyroxine concentrations were measured by radioimmunoassay in plasma samples from 691 lactating ewes in 26 areas of New South Wales, Queensland, Western Australia and Tasmania. Sheep sampled in New South Wales and Tasmania had significantly lower plasma thyroxine values (4.0 and 4.3 microgram/100 ml respectively) than those sampled in Queensland and Western Australia (5.4 and 5.3 microgram/100 ml respectively). However, sheep in some districts in southern Queensland also had low plasma thyroxine values. The areas where sheep had low plasma thyroxine values correlate well with areas where goitre has been previously reported, both in man and in domestic animals. This suggests that measurement of plasma thyroxine is probably a valid empirical method of assessing the relative iodine deficiency of grazing sheep and further that sheep grazing substantial areas of New South Wales, Tasmania and to a lesser extent Queensland may have thyroid dysfunction of varying degrees of severity. These findings could have implications for animal production in these areas.