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Hypothyroidism

Jreptile
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Introduction

  • Hypothyroidism is characterized by impaired production or secretion of thyroid hormones by the thyroid gland, resulting in a decreased concentration of circulating thyroid hormone and associated clinical signs.
  • Primary hypothyroidism has been described in giant tortoises such as the African spurred tortoise African spurred tortoise (Centrochelys [formerly Geochelone] sulcata), Galapagos tortoise (Geochelone nigra [formerly Geochelone elephantopus]) & Aldabra tortoise (Geochelone gigantea). It has not been reported clinically in snakes, lizards or crocodilians.
  • Thyroid gland enlargement (goiter) is seen with reptiles ingesting diets that are low in iodine (hypoiodinism) or high in goitrogens. This has been reported in giant tortoises, an eastern diamondback rattlesnake, (Crotalus adamanteus), a Green iguana Green iguana, (Iguana iguana), and Kirtland’s snakes, (Clonophis kirtlandii).
  • Sick euthyroid syndrome is common with animals with a non-thyroidal illness leading to low thyroid hormone levels. Examples include starvation, poor husbandry, septicemia etc.
  • Hypothyroidism is also described secondary to treatment for hyperthyroidism Hyperthyroidism in a gecko.
  • In mammals a form of hypothyroidism is associated with low selenium diets. This has not been described in reptiles.
  • Cause: unknown.
  • Signs: swelling in neck, myxedema (non-pitting edema), lethargy, decreased appetite.
  • Diagnosis: T4, T3 and free T4 levels on bloods. Fine needle aspirate of myxedema.
  • Treatment: levothyroxine supplementation - good response to levothyroxine treatment has been reported in a male African spurred tortoise.     
  • Prognosis: unknown, very few reported cases in literature.
  • Incidence: although the disease is one of the most common endocrine diseases seen in dogs, hypothyroidism has been described infrequently in reptiles.

Relevant anatomy and physiology

  • Chelonia and snakes have an unpaired, roughly spherical thyroid gland which lies cranial to the heart.
  • Some crocodilians have a bi-lobed gland connected by a narrow isthmus of tissue. Other crocodilians have two separate lobes situated laterally to each bronchi.
  • Lizards show large diverse anatomy, depending on species, having single broad, round or long gland, bi-lobed or paired.
  • Unlike mammals the parathyroid glands are separate to the thyroid glands.
  • Many external factors affect thyroid levels such as season, photoperiod, temperature, nutrition, reproductive, etc status making normal reference values difficult.   

Pathogenesis

Etiology

  • Currently unknown.
  • Etiologies have been suggested, as in dogs, resulting from either immune-mediated lymphocytic thyroiditis or idiopathic thyroid atrophy for primary hypothyroidism.
  • Low iodine diets: this can be difficult to assess as many species are highly adapted to their environment and their food or prey species may also be deficient in captivity. Problems can be exacerbated by feeding goitrogenic diets. Care must be taken in iodine supplementation as toxicity can occur.
  • High goitrogen diet: food that contains high levels of thiocyanates such as: cabbage, mustard greens, collard greens, bok choy, broccoli, brussels sprouts, cauliflower, kale, mustard seed, rape seed and turnips are considered goitrogenic. These compounds interfere with normal uptake of iodine by the thyroid gland. The pituitary gland secretes increased thyrotropin in response, and this results in a goiter or hypertrophy of the thyroid gland.
  • Other causes could include post thyroidectomy, over-supplementation of treatment for hyperthyroidism Hyperthyroidism, post-radioactive iodine treatment.

Predisposing factors

General

  • Low iodine diets and concurrent high goitrogen containing foods.

Specific

  • Species that have evolved to live in volcanic environments, such as tortoises on the Galapagos Islands, seem more susceptible to hypothyroidism. This is because plants that contain large levels of halogens (such as iodine and bromine) are plentiful in these regions.

Pathophysiology

  • Currently not known.

Timecourse

  • Usually, a slow chronic onset over weeks.

Epidemiology

  • Hypothyroidism is rare in reptiles.

Diagnosis

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Treatment

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Pajdak-Czaus J, Terech-Majewska E, Będzłowicz D et al (2019) Applicability of thyroxine measurements and ultrasound imaging in evaluations of thyroid function in turtles. J Vet Res 63 (2), 267-273 PubMed.
  • Mancinelli E (2015) Overview of common nutritional disorders of captive reptiles. Vet Times.
  • Mans C & Braun J (2014) Update on common nutritional disorders of captive reptiles. Vet Clin North Am Exot Anim Pract 17 (3), 369-395 PubMed.
  • Hadfield C, Clayton L, Clancy M, Beck S, Mangus L & Montali R (2012) Proliferative thyroid lesions in three diplodactylid geckos: nephrurus amyae, nephrurus levis, and oedura marmorata. J Zoo Wildl Med 43 (1), 131-140.
  • Franco K H & Hoover J P (2009) Levothyroxine as a treatment of presumed hypothyroidism in an adult male African spurred tortoise (Centrochelys sulcata). J Herpetol Med Surg 19 (2), 42-44 VetMedResource.
  • Franco K H, Famini D J, Hoover J P & Payton M E (2009) Serum thyroid values for African spurred tortoises (Centrochelys [formerly Geochelone] sulcata). J Herpetol Med Surg 19 (2), 47-49 VetMedResource.
  • Rivera S & Lock B (2008) The reptilian thyroid and parathyroid. Vet Clin Exot Anim 11, 163-175 PubMed.
  • Gyimesi Z S, Garner M M & Burns R B (2008) Goiter and thyroid disease in captive kirtland’s snakes, clonophis kirtlandii. J Herpetol Med Surg 18 (3/4), 75-80 VetMedResource.
  • Gyimesi Z S, Garner M M & Burns R B (2008) Goiter and thyroid disease in captive kirtland’s snakes, Clonophis kirtlandii. J Herpetol Med Surg 18 (3), 75-80 VetMedResource.
  • Fleming G J, Heard D J, Uhl E W & Johnson C M (2004) Thymic hyperplasia in subadult Galapagos tortoises, Geochelone nigra. J Herpetol Med Surg 14 (1), 24-27 PubMed.
  • Hernandez-Divers S J, Knott C D & MacDonald J (2001) Diagnosis and surgical treatment of thyroid adenoma-induced hyperthyroidism in a green iguana (Iguana iguana). J Zoo Wildl Med 32 (4), 465-475 PubMed
  • Kohel K A, MacKenzie D S, Rostal D C, Grumbles J S & Lance V A (2001) Seasonality in plasma thyroxine in the desert tortoise, Gopherus agassizii. Gen Comp Endocrinol 121 (2), 214–222 PubMed.
  • Raiti P & Haramati N (1997) Magnetic resonance imaging and computerized tomography of a gravid leopard tortoise (Geochelone pardalis pardalis) with metabolic bone disease. J Zoo Wildl Med 28 (2), 189-197 PubMed.
  • Topper M J, Latimer K S, McManamon R & Thorstad C L (1994) Colloid goiter in an Eastern diamondback rattlesnake (Crotalus adamanteus). Vet Pathol 31 (3), 380-382 PubMed.
  • Laforgia V, Cavagnuolo A, Varano L, Putti R, Capaldo A & Sciarrillo R (1992) Endocrinologic effects of the anaesthetic propfol. Ital J Anat Embryol 97 (1), 45-53 PubMed.
  • Licht P, Denver R J, Pavgi S & Herrera B (1991) Seasonality in plasma thyroxine binding in turtles. J Exp Zool 260 (1), 59–65.
  • Licht P, Denver R J & Herrera B (1991) Comparative survey of blood thyroxine binding proteins in turtle. J Exp Zool 259 (1), 43-52 PubMed.
  • Norton T M, Jacobson E R, Caliguri R & Kollias G V (1989) Medical management of a Galapagos tortoise (Geochelone elephantopus) with hypothyroidism. J Zoo Wildl Med 20, 212-216
  • Chiu K W, Sham J S K, Maderson P F A & Zucker A H (1986) Interaction between thermal environments and hormones affecting skin-shedding frequency in the tokay (Gekko gecko) (Gekkonidae, Lacertilia). Comp Biochem Physiol 84 (2), 345-351 PubMed.
  • Chiu K W, Leung M S & Maderson P F A (1983) Thyroid and skin-shedding in the rat snake (Ptyas korros). J Exp Zool 225 (3), 407-410 PubMed.
  • Sawin C T, Bacharach P & Lance V (1981) Thyrotropin-releasing hormone and thyrotropin in the control of thyroid function in the turtle, Chrysemys picta. Gen Comp Endocrinol 45 (1), 7-11 PubMed.
  • Wong K L & Chiu K W (1974) The snake thyroid gland I. seasonal variation of thyroidal and serum iodamino acids. Gen Comp Endocrinol 23 (1), 63-70 PubMed.
  • Frye F L & Dutra F R (1974) Hypothyriodism in turtles and tortoises. Vet Med Small Anim Clin 69 (8), 990-993 PubMed.
  • Chiu K W & Lynn W G (1970) The role of the thyroid in skin-shedding in the shovel-nosed snake, Chionactis occipitalis. Gen Comp Endocrinol 14 (3), 467-474 PubMed.
  • Chiu K W, Philips J G & Maderson P F (1969) Seasonal changes in the thyroid gland in the male cobra, Naja naja L. Biol Bull 136 (3), 347-354 PubMed.
  • Lynn G W (1960) Structure and functions of the thyroid gland in reptile. Am Mid Natural 64 (2), 309-326.
  • Greer M A (1957) Goitrogenic substances in food. Am J Clin Nutr 5 (4), 440–444 PubMed.

Other sources of information

  • Boyer T H & Steffers Z J (2011) Reptilian Thyroid Anatomy, Physiology, and Disease.  In: Proc Assoc Reptilian Amphibian Veterinarians. pp 18-35.
  • Boyer T H, Wallack S, Bettencourt A & Bourdon M (2010) Hyperthyroidism in a Leopard Gecko, Eublepharis Macularius, and Radioiodine (I-131) Treatment. In: Proc ARAV. pp 53.
  • Buehler I (2006) Thyroid Parameters in Common Veterinary Practice Presented in Tortoise Species. Veterinary Doctorate Dissertation, Veterinary Faculty, Ludwig-Maximilians-University, Germany. pp 111.
  • DiGesualdo C L, West G, Brown T R & Hoover J P (2004) Determining Normal Thyroid Hormone Status in Galapagos Tortoises, then Comparing Normal Levels to Thyroid Levels of Galapagos Tortoises (Geochelone Elephantopus) Suspected of Hypothyroidism. In: Proc AAZV, AAWV, WDA Joint Conference. pp 550–551.
  • Donoghue S (2006) Nutrition. In: Reptile Medicine and Surgery. Ed: Mader D R. 2nd edn. Saunders Elsevier, USA. pp 290–291.
  • Griffin C (2006) Possible Thyroid Hyperplasia in a Green Iguana. In: Proc ARAV. pp 38.
  • Lynn G W (1970) The Thyroid. In: Biology of the Reptilia. Eds: Gans C & Parsons J. Academic Press, UK. pp 201-234.

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