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
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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
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
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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.
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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
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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
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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
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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.
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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