Determining Normal Thyroid Hormone Status in Galapagos Tortoises, Then Comparing Normal Levels to Thyroid Levels of Galapagos Tortoises (Geochelone elephantopus) Suspected of Hypothyroidism
Introduction
The number of suspected hypothyroid cases occurring in giant tortoise species is increasing with the numbers being kept in captivity. Clinical signs reported in these animals have included anorexia, lethargy, myxedema of subcutaneous tissues, and fibrous goiter.1 Goiter is most likely a misnomer given the location of the thyroid gland. In turtles and snakes the gland lies ventral to the trachea and just anterior to the heart.4 The cause of these clinical signs has been associated with hypothyroidism secondary to inappropriate husbandry and diet.1,5 However, there have been no clinical studies done to confirm this diagnosis.
Methods
The Oklahoma City Zoo maintains an established group of five clinically normal adult Galapagos tortoises, two male and three female. Each animal had blood collected from the brachial vein 4–6 times over a 6-month period for a total of 22 blood samples. This group of animals was used to establish total T4 reference ranges: 4.74–26.5 nmol/L, mean=10.39 nmol/L, standard deviation=18.72 nmol/L.
The Oklahoma City Zoo received three Galapagos tortoises back from a loan in early 2002. These animals were approximately 10 years old and on return to the Oklahoma City Zoo displayed the characteristic myxedema associated with tortoises suspected of being hypothyroid. These animals were bled three times over a 6-month period for a total of 12 blood samples. The total T4 levels of these three tortoises were: 2.43–6.02 nmol/L, mean=3.94 nmol/L, standard deviation=1.06 nmol/L.
Initial analysis of the data indicates a statistically significant difference between the T4 levels of the assumed normal tortoises and the assumed hypothyroid tortoises (p<0.05). Total T4 levels for all tortoises were determined by the Veterinary T4 ELISA kit (Oxford Biomedical Research, Rochester Hills, MI, USA).
Results and Discussion
Thyroid hormones are involved in the regulation of nutrient assimilation, metabolism, calorigenesis, growth and development, and reproduction.3 There are various factors that can affect the normal function of the reptilian thyroid gland such as age, sex, diurnal changes, seasonal changes, day length, diet, environment, breeding season, exposure to sunlight, and stress.2,3,6 Because of the diversity of things that can affect thyroid hormones in reptiles, determination of thyroid levels by point sampling may not accurately diagnose thyroid status; however, having established normal ranges is an essential first step in the diagnosis of thyroid disease.2
Literature Cited
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2. Greenacre CB, et al. Validation of a novel high-sensitivity radioimmunoassay procedure for measurement of total thyroxine concentration in psittacine birds and snakes. AJVR. 2001;62:1750–1754.
3. Kohel KA, et al. Seasonality in plasma thyroxine in the desert tortoise, Goperus agassizii. Gen Comp Endocrin. 2001;121:214–222.
4. Lynn GW. The thyroid. In: Gans C, ed. Biology of the Reptilia. 1970:201–234.
5. Norton TM, et al. Medical management of a Galapagos tortoise (Geochelone elephantopus) with hypothyroidism. J Zoo Wildl Med. 1989;20:212–216
6. Stamper DL, et al. Effects of thyroidal status on metabolism and growth of juvenile turtles, Pseudemys scripta elegans. Comp Biochem Physiol. 1990;96A(1):67–73.