Plasma thyroxine concentrations in farmed ostriches in relation to age, body weight, and growth hormone.

Thyroid hormones are of interest in ostriches because, in common with other ratites, ostriches are believed to have evolved through neoteny, a process which, in amphibia, is related to thyroid function. Farmed ostriches show marked differences in growth rates, a problem which also could be related to abnormal thyroid function. In adult farmed ostriches (more than 3 years old), mean plasma thyroxine concentration was 1.8 nmol middle dot liter-1 (range 0.2 to 6.5 nmol middle dot liter-1). This is a lower mean and a much greater range than those in adult starlings or Japanese quail measured at the same time. No sex differences were observed. In 5-month-old ostriches (approximately half grown) mean levels were again low (3.1 nmol x liter-1, range 0.2 to 9.9 nmol x liter-1). There was a correlation (P < 0.0005) between thyroxine and body weight, which ranged from 10.8 to 51.5 kg. Growth hormone in this group ranged from 0.7 to 45.6 microg x liter-1, but there was no correlation with body weight or with thyroxine. In the same group of birds at 10 months of age, plasma thyroxine concentrations were similar, but were not correlated with body weight. In young birds sampled between hatch and 13 weeks, mean thyroxine decreased from 7.6 nmol x liter-1 soon after hatching to less than 2 nmol x liter-1 at 2 weeks and remained less than 2 nmol x liter-1 for the following 3 months. Thyroxine was highly variable within and between individuals. There was no correlation with body weight or growth rate. There were no significant differences between values at 13 weeks, 5 months, 10 months, or in adults. The results do not suggest that slow growth is directly related to low thyroxine. However, the low means and wide range of values do suggest that thyroid function in the ostrich is abnormal compared to other birds.