Top Banner
1 Avian Thyroid Metabolism and Diseases As in mammals, synthesis of thyroid hormones occurs in birds when iodide is concentrated within the thyroid gland by the maintenance of a gradient over that of blood. 1 Iodide is converted to I 2 and then I + . Iodine concentration in the avian thyroid peaks at 6h and is then stored for several days. 2 Avian thyroglobulin [TG] is highly iodinated, the iodine representing 1.5% TG by weight. 3 Within the thyroid a peroxidase system converts iodide to iodine with a second enzyme system combining iodinated tyrosines with TG to form triiodothyronine [T 3 ] and thyroxine [T 4 ]. 1 Intrathyroidal iodination and deiodination occur continually, leading to randomization of thyroidal iodine, which shifts between tyrosine and thyronine randomly. 4 Control of thyroid hormone production is via a negative feedback loop regulated by the hypothalamus and adenohypophysis. 1 A decrease in circulating hormone [for any reason] below metabolic requirements stimulates the anterior pituitary to release thyroid stimulating hormone [TSH]. Exogenous administration of T 4 leads to a decrease in TSH. Secretion of TSH is controlled by the hypothalamus via the action of thyroid releasing hormone [TRH]. In adult chickens TRH does not cause TSH release, 5 rather its release from the anterior pituitary is controlled by T 3 . In birds, TSH is mainly thyrotrophic, with no influence on peripheral activation of T 4 -T 3 . 6 Follicle stimulating hormone [FSH] is less effective than TSH, but can increase follicular diameter and epithelial cell height in baby chickens. 7
13

The Avian Thyroid Gland

Dec 16, 2022

Download

Documents

Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: The Avian Thyroid Gland

1

Avian Thyroid Metabolism and Diseases

As in mammals, synthesis of thyroid hormones occurs in birds when iodide

is concentrated within the thyroid gland by the maintenance of a gradient over that

of blood.1 Iodide is converted to I2 and then I+. Iodine concentration in the avian

thyroid peaks at 6h and is then stored for several days.2

Avian thyroglobulin [TG] is highly iodinated, the iodine representing 1.5% TG

by weight.3 Within the thyroid a peroxidase system converts iodide to iodine with a

second enzyme system combining iodinated tyrosines with TG to form

triiodothyronine [T3] and thyroxine [T4].1

Intrathyroidal iodination and deiodination occur continually, leading to randomization

of thyroidal iodine, which shifts between tyrosine and thyronine randomly.4

Control of thyroid hormone production is via a negative feedback loop

regulated by the hypothalamus and adenohypophysis.1 A decrease in circulating

hormone [for any reason] below metabolic requirements stimulates the anterior

pituitary to release thyroid stimulating hormone [TSH]. Exogenous administration of T4

leads to a decrease in TSH. Secretion of TSH is controlled by the hypothalamus via

the action of thyroid releasing hormone [TRH]. In adult chickens TRH does not cause

TSH release,5 rather its release from the anterior pituitary is controlled by T3. In birds,

TSH is mainly thyrotrophic, with no influence on peripheral activation of T4-T3.6 Follicle

stimulating hormone [FSH] is less effective than TSH, but can increase follicular

diameter and epithelial cell height in baby chickens.7

Page 2: The Avian Thyroid Gland

2

Under the influence of TSH, cyclic 3', 5' adenosine monophosphate [cAMP] is

activated in thyroid follicular epithelial cells. Increased cAMP leads to increased iodide

trapping from the blood by the follicular cells.8 As a result follicular colloid is processed

into follicular epithelial cells leading to release of thyroid hormones into the circulation.

Approximately 60% is T4, and 40% T3.

Thyroid hormones and growth hormone[GH] are related in birds, as thyroid

hormones inhibit synthesis and release of GH.9 This may be a feedback mechanism

as GH enhances peripheral production of T3 but not T4. Growth hormone appears to

impair T4 release as well as stimulating its monodeiodination. 5,9

In the liver T4 and T3 are metabolized due to the action of I 5' deiodinase.1,10

Growth hormone inhibits type III deiodinating enzyme in the liver.

Thyroid hormones control metabolism and development with the actions

mediated by nuclear thyroid hormone receptors that have their highest affinity for T3. In

birds, the development of thermogenic metabolic responses is correlated with the

pattern of thyroid development and extrathyroidal deiodinations of thryroid hormones.11

Environmental and Other Influences on Thyroid Function

The secretion rate of T4 in young chicks and adult chickens in the summer is

1/2 of the winter rate.1 This effect is directly correlated to ambient temperature. In

winter the follicular cells are higher and follicular volume is greater.12

The photoperiod also affects thyroid function and metabolism. A long

photoperiod depresses I131 uptake in ducks and quail.1 Long days inhibit thyroid

Page 3: The Avian Thyroid Gland

3

function but stimulate thyrotrophic activity. During daylight T4 is depressed and T3

elevated, while the reverse occurs at night.13

Thyroid hormone is involved in maintaining photorefractoriness in turkey

hens.14 Light intensity affects food conversion efficiency in turkey males with body

weight highest under the lowest light intensity coinciding with higher weight gain and

lower food intake.15

Food intake and diet composition is also related to thyroid hormone

concentration and metabolism. The T4/T3 ratio increases during fasting, and plasma

TSH concentration decreases.16,17 Hypothalamic TRH content is elevated, suggesting

a decreased hypothalamic release. In addition, fasting leads to increased hepatic type

III deiodinase [D3] and decreased renal D3 activity. No change was noted in either

hepatic or renal type I deiodinase [D1].18

The level of dietary protein fed to chickens had an effect on the concentrations

of T4 and T3 after adding T3 to the bird's diet. With a high level of protein, T4 was

greater and T3 less, however the protein level did not change the response to the

addition of T3 to the diet, indicating that dietary protein levels may not affect adaptive

responses to T3.19

The affect may be due to selected amino acid deficits.20 When chicks are fed a

restricted diet there is a decrease in plasma T3. Arginine deficiency prevents this

decrease, but does not alter T4 concentration suggesting that there is a specific

alteration in the metabolism of T3. Methionine deficiency also leads to elevated

plasma T3 concentration in diet-restricted chicks.

Page 4: The Avian Thyroid Gland

4

Other amino acids with similar effects include lysine and isoleucine. Leucine

and threonine have no effect on plasma T3. Lysine is the only amino acid that lowered

T4.

The level of selenium affects the growth of chickens via thyroid hormone

metabolism.21 Dietary selenium supplementation increases plasma T3, while T4 was

decreased. Hepatic 5' deiodinase activity is elevated by selenium, and a selenium

deficiency can depress growth by inhibiting 5' deiodinase activity causing lower

plasma T3 concentration.

Relationship to Moulting/Feather Growth

In penguins changes in T3 were not shown to be consistent with moulting, but

an increase in T4 associated with a decrease in plasma levels of sex steroid

hormones induce moulting.22 In thyroidectomized birds [spotted munia] T4 is more

effective than T3 at inducing feather regeneration.23 In chickens, doses of T4 at 0.2

mg/bird diminished egg production but did not result in moulting, while doses of 0.4

mg/bird caused feather loss after day 14 of administration.24 In general, an increase in

thyroid hormones leads to moulting, possibly by stimulating new feather growth.25

Thyroid Disease

Page 5: The Avian Thyroid Gland

5

Diseases of thyroid are well documented in chickens, but less well described in

other avian species. Cystic thyroids are occasionally seen, but the cysts may actually

arise in the ultimobrancial body.26

The thyroid gland may play a part in humoral immunity, as physiologic

levels of thyroid hormone are necessary to maintain normal weights of the spleen and

bursa of Fabricius.27 The thyroid also is involved in autoimmune disease in obese

strain chickens, which get a spontaneous autoimmune thyroiditis.28,29 The condition

develops in the first 2-3 weeks post-hatching. As in humans, the iodine content of the

obese chicken thyroglobulin was lower than that of normal chickens, however these

chickens have almost no inorganic iodide in their thyroid glands. A morphologically

similar condition has been seen in African grey parrots.30

The most common avian thyroid problem reported is hyperplasia [goiter].31

Although the literature indicates that the condition is most common in budgerigars, a

study of 16 years of our data indicate that macaws, particularly blue and gold

macaws, have the highest incidence of hyperplastic goiter.32 Colloid goiter is also

seen sporadically33 Clinical signs of thyroid hyperplasia may be absent, or are

referable to bilateral enlargement of the thyroid glands that put pressure on

surrounding tissues, including the trachea. The primary cause of goiter in budgerigars

has been considered to be iodine deficiency,31 but other potential causes include the

feeding of goitrogenic substances of plant origin, and genetically induced biosynthetic

problems.32

Morphologically hyperplastic goiter results in symmetrical enlargement of the

thyroid glands.34 Affected glands are red-brown and firm. Histologically there is a

Page 6: The Avian Thyroid Gland

6

decrease or complete lack of follicular colloid, and the follicular epithelial cells are

cuboidal or low columnar.

Functional hypothyroidism is considered uncommon in birds,35 although one

case in a scarlet macaw was documented. 36

An experimental model of hypothyroidism was developed in cockatiels,37

however classical clinical and laboratory signs such as poor feathering and

hypercholesterolemia were either absent or mild 48 days following

radiothyroidectomy. This may indicate that a longer course of the disease is

necessary for these signs to develop. Without TSH response testing or histologic

confirmation of lesions associated with clinical hypothyroidism the true incidence of

hypothyroidism is difficult to determine.

In older, obese amazon parrots lesions of the thyroid have been associated

with atherosclerosis and hypothyroidism was suspected by not confirmed.31 In our

series32 hypothyroidism was considered a possible stress factor leading to other

diseases, but it was not proved by appropriate clinical testing.

Thyroid neoplasia, including adenoma and carcinoma, is frequently reported in

budgerigars,38 although also seen in other species.33 Histologically adenomas are

comprised of large cuboidal epithelial cells that form follicular and papillary structures,

as well as cystic spaces. Carcinomas are similar but more anaplastic and invasive.39

The tumors usually are not functionally secreting.31

Thyroid Function Tests

Page 7: The Avian Thyroid Gland

7

Thyroid scintigraphy was done in normal and radiothyroidectomized cockatiels,

using sodium 99mTechnetium pertechnetate. Scintigraphy was capable of detecting

hypofunctional thyroid abnormalities in this study.40 This type of testing is a research

tool however and not readily available to practitioners.

The best method of testing the avian thyroid for abnormalities is the

administration of TSH and measuring the serum T4 concentration after a particular

time period.41,42 Unfortunately since veterinary TSH is no longer on the market, the

test must be done with an expensive human product.43

The alternative is measurement of T4 only.44 Since avian T4 is lower than

mammalian, the test must be able to lower T4 values. A single low T4 test cannot lead

to a definitive diagnosis of hypothyroidism, and conversely a single normal may not be

proof that the bird is not hypothyroid. The test should be interpreted in association

with multiple factors including clinical signs and other tests.

Recently a method for using a high-sensitivity radioimmunoassay to measure

total thyroxine concentration was developed. 45 This method was developed using

psittacine birds only. In these birds it will be a way to measure T4 concentrations.

Results of the study indicated that T4 concentrations in blue-fronted Amazon parrots

were higher and more variable than in other species tested.

Page 8: The Avian Thyroid Gland

8

References

1. Wentworth BC, Ringer RK: Thyroids, in Sturkie PD [Ed]: Avian Physiology. New York,

NY, Springer-Verlag, 1985, pp452-465

2. Newcomer WS: Dietary iodine and accumulation of radioiodine in thyroids of chickens.

Am J Physiol 234: 168-176,1978

3. Hoshiro T, Ui, N: Comparative studies on the properties of thyroglobulins from various

animal species. Endocrinol Jpn 17: 521-533, 1970

4. Newcomer WS: Accumulation of radioiodine in thiouracil-hyperplastic thyroids of

chicks. Am J Physiol 237: 147-151, 1979

5. Kuhn ER, Berghman LR, Moons L, et. al.: Hypothalamic and peripheral control of

thyroid function during the life cycle of the chicken. In Sharp PJ [ed]: Avian

endocrinology. London, Burgess Science, 1993, pp29-46

6. Singh PP, Pradeep Kumar G, Laloraya M: Regulation of superoxide anion radical-

superoxide dismutase system in the avian thyroid by TSH with reference to thyroid

horomogenesis. Biochem Biophys Res Com 239: 212-216, 1997

7. Dobozy O, Balkanyi L, Csaba G: Overlapping effect of thyroid stimulating hormone and

follicle stimulating hormone on the thyroid gland in baby chickens. Acta Physiol Acad

Sci Hung 57: 171-175, 1981

Page 9: The Avian Thyroid Gland

9

8. Larsen PR, Davies TF, Hay ID: The thyroid gland. In: Wilson JD, Foster DW [ed]:

Williams Textbook of Endocrinology, 9th ed. Philadelphia, WB Saunders, 1998, pp390-

515

9. Hull KL, Janssens WCJ, Baumbach WR, Harvey S: Thyroid glands: novel sites of

growth hormone action. J Endocrinol 146: 449-458, 1995

10. Kohrle J, Brabant G, Hesch RD: Metabolism of thyroid hormones. Horm Res 26: 58-78,,

1987

11. McNabb FM: Thyroid hormones, their activation, degradation and effects on

metabolism. J Nutr 125[6 suppl]: 1773S-1776S, 1995

12. Breit S, Konig HE, Stoger E: The morphology of the thyroid gland in poultry with

special regard to seasonal variations. Anat Histol Embrol 27: 271-276, 1998

13. Klundorf H, Lea W, Sharp PJ: Thyroid function in layng, incubating and broody bantam

hens. Gen Comp Endocrinol 47: 492-496,1982

14. Siopes T: Transient hypothyroidism reinitiates egg laying in turkey breeder hens:

Termination of photorefractoriness by propylthiouracil. Poult Sci 76: 1776-1782, 1997

15. Yahav S, Hruwitz S, Rozenboim I: The effect of light intensity on growth and

development of turkey toms. Br Poult Sci 41: 101-106, 2000

16. May JD, Reece FN: Relationship of photoperiod and feed intake to thyroid hormone

concentration. Poult Sci 65: 801-806, 1986

17. Geris KL, Berghman LR, Kuhn ER, Darras VM: The drop in plasma thyrotropin

concentrations in fasted chickens is caused by an actin at the level of the

hypothalamus: role of corticosterone. Domest Anim Encocrinol 16: 231-237, 1999

Page 10: The Avian Thyroid Gland

10

18. Van der Geyten S, Van Rompaey E, Sanders JP, et al: Regulation of thyroid hormone

metabolism during fasting and refeeding in chicken. Gen Comp Endocrinol 116: 272-

280, 1999

19. Rosebrough RW, McMurtry JP, Vasilatos-Younken R: Dietary protein effects on the

broiler's adaptation to triiodothyronine. Growth Dev Aging 63: 85-98, 1999

20. Carew LB, Evarts KG, Alster FA: Growth and plasma thyroid hormone concentrations

of chicks fed diets deficient in essential amino acids. Poult Sci 76: 1398-1404

21. Jianhua H, Ohtsuka A, Hayashi K: Selenium influences growth via thyroid hormone

status in broiler chickens. Br J Nutr 84: 727-732, 2000

22. Otsuka R, Aoki K, Hori, H, et al: Changes in circulating LH, sex steroid hormones,

thyroid hormones and corticosterone in relation to breeding and molting in captive

humboldt penguins [Spheniscus humboldti] kept in an outdoor open display. Zoolog Sci

15: 103-109, 1998

23. Pant K, Chandola-Saklani A: Effects of thyroxine on avian moulting may not involve

prior conversion to triiodothyronine. J Endocrinol 137: 265-270, 1993

24. Szelenyi Z, Peczely P: Thyroxin induced moult in domestic hen. Acta Physiol Hung 72:

143-149, 1988

25. King AS, McLelland J: Endocrine system in Birds Their structure and function, London,

Bailliere Tindall, 1985

26. Ito, M., Kameda, Y., Tagawa, T: An ultrastructural study of

the cysts in chicken ultimobranchial glands, with special reference to C-cells. Cell

Tissue Res. 246: 39-44, 1986

Page 11: The Avian Thyroid Gland

11

27. Bachman SE, Mashaly MM: Relationship between circulating thyroid hormones and

humoral immunity in immature male chickens. Dev Comp Immunol 10: 395-403, 1986

28. Sundlick RS, herdegen D, Brown TR, et al: Thyroidal iodine metabolism in obese-

strain chickens before immune mediated challenge. J Endocrinol 128: 239-244, 1991

29. Dietrich HM, Oliveira dos Santos AJ, Wick G: Development of spontaneous

autoimmune thyroiditis in obese strain [OS] chickens. Vet Immunol Immunopathol 57:

141-146, 1997

30. Schmidt RE: Immune system, in Altman RB, Clubb SL, Dorrestein GM, Quesenberry

K [eds]: Avian Medicine and Surgery. Philadelphia. PA, Saunders 1997, pp645-652

31. Rae M: Endocrine disease in pet birds. Sem Avian Exotic Pet Med 4: 32-38, 1995

32. Schmidt RE, Reavill DR: Thyroid hyperplasia in birds. Submitted for publication, J

Avian Med Surg.

33. Wadsworth PF, Jones DM: Some abnormalities of the thyroid gland in non-

domesticated birds. Avian Pathol 8: 279-284, 1979

34. Ivanics E, Rudas P, Salye G, et al: Massive goitre [struma parenchymatosa] in geese.

Acta Vet Hung 47: 217-231, 1999

35. Merryman JI, Buckles EL: The avian thyroid gland. Part two: A review of function and

pathophysiology. J Avian Med Surg 12: 238-242, 1998

36. Oglesbee B: Hypothyroidism in a scarlet macaw. J Am Vet Med Assoc 201: 1599-

1601, 1992

37. Harms CA, Hoskinson JJ, Bruyette DS, et al: Development of an experimental model

of hypothyroidism in cockatiels {nymphicus hollandicus]. Am J Vet Res 55: 399-404,

1994

Page 12: The Avian Thyroid Gland

12

38. Leach MW: A survey of neoplasia in pet birds. Sem Avian Exotic Pet Med 1:52-64,

1992

39. Schmidt RE: Morphologic diagnosis of avian neoplasms. Sem Avian Exotic Pet Med

1: 73-79, 1992

40. Harms CA, Hoskinson JJ, Bruyette DS, et al: Technetium-99m and Iodine-131 thyroid

scintigraphy in normal and radiothyroidectomized cockatiels [Nymphicus hollandicus].

Vet Radiol ultrasound 35: 473-478, 1994

41. Zenoble RD, Kemppainen RJ, Young DW, et al: Endocrine response of healthy

parrots to ACTH and thyroid stimulating hormone. J Am Vet Assoc 187: 1116-1118,

1985

42. Lothrop CD, Loomis MR, Olsen JH: thyrotropin stimulation test for evaluation of

thyroid function in psittacine birds. J Am Vet ASSoc 186: 47-48, 1985

43. Fudge AM, Speer B: Selected controversial topics in avian diagnostic testing. Sem Av

Exotic Pet Med 10: 96-101, 2001

44. Rae M: Avian endocrine disorders, in Fudge Am [ed]: Laboratory medicine. Avian and

exotic pets. Philadelphia, PA, Saunders,2000, pp76-89

45. Greenacre CB, Young DW, Behrend EN, Wilson GH: Validation of a novel high-

sensitivity radioimmunoassay procedure for measurement of total thyroxine concentration

in psittacine birds and snakes. AJVR 62: 1750-1754, 2001

Page 13: The Avian Thyroid Gland

13