Thyroid and Parathyroid Function and Laboratory
Assessment
Serkan SAYINER, DVM PhD. Assist. Prof.
Near East University, Faculty of Veterinary Medicine, Department of Biochemistry
serkan.sayiner@neu.edu.tr
Thyroid Gland
Glandula thyroidea
Thyroid Gland
▪The thyroid gland of animals is a bilobed structure that overlays the trachea at a point just below the larynx.
▪Anatomical variations of the gland are marked between species and, to some extent, within a given species.
• In the horse, sheep, goat, cat, and dog, the isthmus is a narrow remnant of tissue and may be nonexistent.
• The gland is extremely vascular.
• The thyroid gland is also called ‘the regulator of all body functions’.
Thyroid Gland
▪The thyroid gland is a highly vascularized tissue with a large blood flow. The functional unit of the thyroid gland is the thyroid follicle.
• Colloid is a thyroglobulin-hormone complex that is the storage reservoir of thyroid hormone in the thyroid gland.
• The colloid stored in the lumen is a clear, viscus fluid.
▪Interspersed between the follicles are the thyroid “C” cells, the source of calcitonin, the hypocalcemic hormone
associated with calcium metabolism.
Thyroid Gland
▪The thyroid gland is unique among the endocrine glands in that an integral part of its hormone, L- thyroxine (T
4), is a trace mineral, iodine, which is available to the animal in only limited amounts.
• The main functions of the thyroid gland are the trapping of I-
and the synthesis, storage, and release of thyroid hormones and these activities are under the control of the thyrotropin or the thyroid-stimulating hormone (TSH).
• Although TSH stimulates all steps in hormonogenesis, the
trapping of I- and the release of hormone are the two major sites of its action.
Thyroid Function
▪The principal thyroid hormones elaborated by the thyroid are the two active hormones,
• 3,5,3’,5’ -tetraiodothyronine (thyroxine, T4)
• 3,5,3’-triiodothyronine (T3),
• The inactive hormone, 3,3’,5’-triiodothyronine (reverse T3 or rT3). The rT3 is the inner deiodination product of the T4 .
▪The T
4functions as the transport form and as the feedback
regulator of the thyroid gland. The T
3is the active hormone
in the target cell.
Thyroid Function
▪The thyroid hormones in the circulation are T
4, T
3, and rT
3. Immediately upon entering the circulation, these hormones are bound to transport proteins, mainly to thyroxine-
binding globulin (TBG), and with lesser amounts to
thyroxine binding prealbumin (TBPA) and to albumin.
• TBPA is present in all the species in contrast to earlier reports indicating that TBPA was present only in humans, rhesus
monkey, horse, cat, rabbit, pigeon, and chicken.
Thyroid Function
▪TBG is the major binding protein for hormone, but not all species have TBG. In those species without TBG,
albümin serves as the major binding protein. Therefore, TBG or albümin transports most of the hormones.
• In the cat, rabbit, rat, mouse, guinea pig, pigeon, or chicken, TBG is absent and most of the hormone is transported by albumin.
Thyroid Hormone Effects
Clinical Tremors, nervousness, exophthalmos, hyperactivity, weight loss
Physiological Increased temperature, heart function Calorigenic Increased basal metabolic rate
(O2 consumption)
Carbohydrate metabolism Increased glucose turnover, absorption Protein metabolism Anabolic, positive N balance
Lipid metabolism Decrease in blood cholesterol
Development Stimulation of growth and maturation Reproductive Fertility, pregnancy, ovulation
Hematological Erythropoiesis
Thyroid Function - IndirectTests
▪Hematology
• A moderate normocytic normochromic anemia is sometimes associated with clinical hypothyroidism in the dog.
▪Cholesterol
• The serum cholesterol generally varies inversely with thyroid activity.
• The net effect of thyroid hormone on cholesterol metabolism is to increase the rate of its catabolism by the liver thereby
lowering the cholesterol.
Thyroid Function Tests
▪Cholesterol (continue)
• Total cholesterol alone is of limited value because
hypercholesterolemia is seen in a variety of conditions unrelated to thyroid activity. These include the diet, nephrotic syndrome, hepatic function, biliary obstruction, and diabetes mellitus.
• The diagnostic accuracy of serum cholesterol for hypothyroidism in the dog is about 66%. However, when the concentrations are very high, >500 mg/ dl ( 12.9 mmol/l), and diabetes mellitus is eliminated, its diagnostic accuracy increases greatly. Therefore, increased cholesterol again is simply a signal to further
investigate thyroid disease.
Thyroid Function - Direct Tests
1. To measure the amount of the hormones in the blood, 2. To assess the response of the thyroid to stimulation
by the thyroid stimulating hormone (TSH), or
3. To assess the response of the pituitary gland to
stimulation by the thyrotropin releasing hormone
(TRH).
Thyroid Function - Direct Tests
▪ Thyroxine (T
4)
• Total T4 (TT4) RIA is widely used in dogs and cats.
▪ Triiodothyronine (T
3)
• It closely parallels T4-RIA in the dog so that the simultaneous determination of T4-RIA and T3-RIA will increase the diagnostic accuracy of either one alone. In cats, T3-RIA is less widely used in comparison to T4-RIA.
▪ Free T
4and Free T
3• Equilibrium dialysis is now considered the best method for
determining the free hormones, but it is often too labor intensive for use in many clinical laboratories. An equilibrium dialysis method for free thyroxine is commercially available so that this method is now widely used.
Thyroid Function - Direct Tests
▪ Thyroid-Binding Globulin, Thyroglobulin, and Thyroid Autoantibodies
• Thyroid-binding globulin (TBG) and thyroglobulin (Tg) or colloid are measured by RIA. The standard technique for thyroglobulin
antibodies (TgAA) is the ELISA method.
• Detected TgAA in a high percentage of dogs with hypothyroidism thyroid autoimmunity is strongly genetically influenced in the dog.
▪ Thyroid-Stimulating Hormone
• ELISA, RIA, Chemiluminescent
▪ Thyroid-Relasing Hormone
• No reliable assay.
Thyroid Function – Response Tests
▪TSH Response Test or Stimulation Test
• The response of the thyroid to TSH injection is a means of evaluating thyroid activity as well as to differentiate a primary hypothyroidism because of a thyroid lesion from a
hypothyroidism secondary to a pituitary lesion.
• In a primary hypothyroidism where the lesion is localized in the thyroid, there is no response to the exogenous TSH.
• If the hypothyroidism is due to a pituitary hypofunction with a deficiency of TSH or a hypothalamic lesion with a lack of TRH, there will be a response of the thyroid to the exogenous TSH as seen by a significant increase in serum hormone concentrations.
Thyroid Function – Response Tests
▪TSH Response Test or Stimulation Test (continue)
• For Dogs;
1. First to obtain a serum sample for baseline T4 or T3 ,
2. Inject 0.1IU/kg bovine TSH i/v. or 100 to 150 ug/dog human recombinant (rh)TSH intravenously.
• It has been also recommended a minimum of 5IU for the dog and horse, respectively.
3. After 4 -6 h, a second serum sample is taken and hormone again measured.
• The normal response in dogs is a doubling or more of the hormone above baseline level.
Thyroid Function – Response Tests
▪TRH Response Test or Stimulation Test
• The response to thyrotropin-releasing hormone (TRH) has been used in dogs and cats. the test had little advantage over using the baseline TSH, total T4, or fT4.
• For Dogs;
1. Take blood sample for basal TT4 concentration.
2. Inject TRH i/v slowly over one minute.
• 1 - 5 kg 100ug TRH; 5 - 30 kg 200ug TRH; >30 kg 300ug TRH
3. Take a second blood sample 4 - 6 hours later for post TT4
concentration. TT4 levels in a normal dog should increase by about 1.2 times the basal concentration
Thyroid Function – Response Tests
▪TRH Response Test or Stimulation Test
• For Cats;
1.Collect basal blood sample.
2.Administer 100 μg/kg i/v slowly over one minute 3.Collect second blood sample 4 hours later
4.Label samples clearly and request TT4.
Normal cats increase 1.5 - 2 times following stimulation.
Thyroid Function – Response Tests
▪ T
3Suppression Test
• For cats; This is a useful test for diagnosing borderline cases of hyperthyroidism where the total T4 is consistently within the normal range. It is very important to measure Total T3 (TT3) as well to ensure the cat actually absorbed the T3 dose.
1.Collect basal blood sample for TT4 and TT3.
2.Administer T3 (Tertroxin) orally every 8 hours for a total of 7 doses according to body weight; cats <5 kg 20 µg of Tertroxin; cats >5 kg 30 µg of Tertroxin
3.Collect second blood sample 2 - 6 hours after final dose 4.Label samples clearly and request total T4 and total T3.
T3 suppression normal cats usually show at least 50% reduction in total T4 levels following suppression.
Thyroid Diseases
▪Goiter
• Goiter may be defined as an enlargement of the thyroid gland, which is not due to inflammation or malignancy. There are two general types of goiters:
1. Nontoxic goiters, which produce either normal amounts of hormone (simple goiter) or below normal amounts of hormone (hypothyroid), and
2. Toxic goiters, which produce excess amounts of hormone (hypertrophy).
• Iodine deficiency (endemic goiter) is well known in iodine-
deficient areas of the world. Goitrogenic materials, either natural substances or drugs, induce goiters by their blocking effects on steps in the hormonogenic pathways.
Thyroid Diseases
▪Hypothyroidism
• Hypothyroidism may be the result of a variety of causative factors.
• Thyroiditis, with similarities to Hashimoto’s thyroiditis in humans, has been reported in about 12% of beagle dogs.
• Antithyroglobulin antibodies were found in these dogs.
• In the adult dog, follicular atrophy and lymphocytic
thyroiditis (primary hypothyroidism) is probably the most common cause of hypothyroidism.
• Finally, hypothyroidism may be secondary to a pituitary insufficiency (secondary hypothyroidism).
Thyroid Diseases
▪Hypothyroidism (continue)
• Some dog breeds are sensitive such as Doberman, Golden Retriever.
• Clinical findings in animals with hypothyroidism may differ. The reason for most clinical findings is related to the decrease in basal metabolic rate.
• The clinical signs of hypothyroidism develops after approximately 75% of the gland is destroyed.
• The hypothyroid dog is typically obese, lethargic, has
myxedema, a dry skin, and a sparse hair coat. Hypothyroidism is therefore an important differential in the diagnoses of
dermatoses.
Thyroid Diseases
▪Hypothyroidism (continue)
• The requirement of T4 for normal reproduction, growth, and
development is well known, so hypothyroidism is an important differential in reproductive failures.
• In the initial screen, an increased cholesterol is often the first clue to hypothyroidism. Definitive laboratory findings in the hypothyroidism of animals are a low T4 or T3 with little or no response to the TSH response test. Therefore, the
recommended algorithm is to first obtain the total T4 and T3 (and the fT4 and fT3 if available). If the results are equivocal, this is
followed by the TSH response test.
Thyroid Diseases
▪Hypothyroidism (continue)
• Silent thyroiditis – antibody-positive; euthyroid
• The thyroid hormones are within reference intervals.
• Subclinical (compensating) thyroiditis
• T4 and T3 antibody-positive; euthyroid (T4 and T3 within reference intervals)
• Endogenous TSH concentration is high
• Clinical disease – antibody-positive; overtly hypothyroid
Thyroid Diseases
▪Idiopathic thyroid atrophy
• Idiopathic thyroid atrophy accounts for most of the remaining
non-thyroiditis cases of hypothyroidism, and is characterized by the following:
• Follicular cell degeneration
• Reduction in follicular size
• Normal parenchymal tissue is replaced with adipose connective tissue, but inflammatory infiltration is not remarkable
Thyroid Diseases
▪Hyperthyroidism
• Hyperthyroidism or toxic goiter is characterized by weight loss, hyperactivity, a voracious appetite, and increased thyroid
hormones.
• Hyperthyroidism is rarely observed in dogs, but in the cat, the high incidence of hyperthyroidism has been recognized as a common endocrinopathy. The most common form of
hyperthyroidism in the cat is a functional thyroid adenoma.
• Suggested that T4 and T3 are sufficient for diagnosis and that the free hormones are not needed for the diagnosis.
Thyroid Diseases
▪Tumors of the Thyroid Gland
• Except for the dog and cat, tumors of the thyroid ocur infrequently in animals.
• About 20% of canine thyroid tumors are functional.
• Scintigraphic imaging is now used to identify functional thyroid tumors and their metastases.
• In feline hyperthyroidism, a functional thyroid adenoma is the most common finding.
Parathyroid Glands
Glandula parathyroidea
Parathyroid Glands
▪The parathyroid gland is imbedded within the thyroid or located in close proximity to it. The parathyroids are the source of parathormone (PTH), the hypercalcemic
hormone.
▪The parathyroid or chief cells primarily respond to calcium.
▪A change in the serum calcium concentration of as little as 5–10% (total calcium 0.25–0.5 mg/dL, or ionized calcium 0.1 mmol/L) can stimulate or inhibit the secretion of
parathyroid hormone.
Parathyroid Glands
▪PTH rarely needs to be measured. The only time it is
“required” is to confirm primary hypoparathyroidism and in some cases of primary hyperparathyroidism.
▪PTH is often within reference intervals in each of these diseases which seems paradoxical, but the PTH value is inappropriate for the serum concentration of calcium.
▪75% of dogs with hypercalcemia due to primary
hyperparathyroidism have serum concentrations of PTH
within reference interval.
Laboratory Tests
▪Total Serum Calcium
• Excellent screening test, included on chemistry panels.
▪Ionized Calcium
• Biologically active fraction; if available, measure in critical care patients no matter what their primary disease or total calcium concentration is; it is excellent for STAT needs. It is more useful in hypocalcemic than hypercalcemic patients. Special sample handling is required.
▪Adjusted Calcium
• Use if hypoalbuminemic; it explains the most common cause of hypocalcemia.
• Adj. Ca = measured Ca + (3.5 – measured Albumin)
Laboratory Tests
▪Calcium-phosphorus product (Ca × P)
• Calcium and phosphorus should be interpreted together in a chemistry panel as the list of differential diagnoses will vary depending on their concurrent values.
• The product of these two electrolytes is predictive of soft tissue mineralization.
• A product of Ca × P > 70 indicates soft tissue mineralization is likely and a product >90 indicates mineralization is occurring.
• Ca 10.2 mg/dL, P 14 mg/dL = 143
• Ca 8.1 mg/dL, P 21 mg/dL = 170
• Ca 15.5 mg/dL, P 1.8 mg/dL = 28
Laboratory Tests
▪Parathyroid hormone (PTH)
• PTH is a peptide hormone; it has a half-life that is so short it is measured in minutes and PTH reacts within minutes to changes in the serum concentration of calcium.
- Ionized calcium is the key signal to calcium receptors on chief cells of the parathyroid gland.
- Calcitriol decreases production and secretion of PTH.
- Hypomagnesemia decreases production and secretion of PTH.
- Epinephrine has a minor influence on PTH secretion.
- Phosphorus directly or indirectly stimulates the opposite effects of calcium.
Laboratory Tests
▪Parathyroid hormone (PTH) (cont.)
• PTH exerts its effect primarily through the binding and activation of PTH receptors, PTH1R.
• Parathyroid hormone related protein (PTHrp), operative in hypercalcemia of malignancy, uses the same PTH1R receptors.
Therefore, it acts identically to native PTH and produces hypercalcemia and hypophosphatemia as seen in
hypercalcemia of malignancy.
• Primary hyperparathyroidism and hypercalcemia of malignancy are the only two diseases that cause hypercalcemia and
hypophosphatemia in dogs and cats.
Laboratory Tests
▪Parathyroid hormone (PTH) (cont.)
• Reference labs have assays for PTH and PTHrp that can be
used on plasma or serum from dogs, cats and horses. Because of peptide stability factors, it is critical to consult the reference lab for specific sampling procedures and interpretive guidelines.
• The most diagnostic information is gained when PTH, PTHrp, and calcium are measured in the same sample or at the same time.
• IRMA, CLIA
Laboratory Tests
▪Parathyroid hormone related protein (PTHrp)
• PTHrP is similar to PTH in structure.
• Causes bone resorption and renal calcium conservation.
• PTHrP is not normally excreted in an adult animal in any appreciable amount.
• If PTHrP is present in any significant amount, malignancy is suspected.
• The absence of detectable PTHrP does not rule out malignancy.
Hypoparathyroidism
▪Spontaneous hypoparathyroidism is due to lymphocytic plasmacytic destruction of parathyroid tissue which
eventually leads to fibrosis, absence of inflammatory cells, and few if any parathyroid cells (sometimes called idiopathic atrophy).
▪The damage may be transient, or permanent and requires life-long treatment with calcium and or vitamin D.
▪Iatrogenic hypoparathyroidism is due to
• thyroidectomy in cats with hyperthyroidism.
• damage or removal during surgery of the neck.
Hypoparathyroidism
▪Moderate to severe hypocalcemia and mild
hyperphosphatemia with normal albumin, urea nitrogen, creatinine and lipase is essentially diagnostic.
▪If the serum calcium is less then 6 mg/dL and the patient is not azotemic and not lactating then primary
hypoparathyroidism is the most likely diagnosis.
▪Ionized calcium can be measured to confirm hypocalcemia
(<1.0 mmol/L) and monitor treatment.
Hypoparathyroidism
▪Concurrent PTH and ionized or total calcium is
measured on the same sample or collected at the same time.
▪Serum PTH will be decreased or undetectable
depending on the stage of the disease. When all the parathyroid tissue is destroyed PTH will be undetectable.
▪if PTH is in reference interval and especially at the low end
of reference interval while there is concurrent and severe
hypocalcemia this is an inappropriate response and is still
diagnostic of primary hypoparathyroidism.
Primary hyperparathyroidism
▪Primary hyperparathyroidism is due to a functional, autonomous secretion of parathyroid hormone
resulting in persistent hypercalcemia and hypophosphatemia.
▪Parathyroid adenomas account for >90% of canine cases of hyperparathyrodisim, carcinoma for less than 5%, and the remainder are considered hyperplasia because a nodule is in more than one gland.
▪Laboratory abnormalities are fairly characteristic.
Primary hyperparathyroidism
▪Total serum calcium will be increased in all dogs with primary hyperparathyroidism. Serum phosphorus is
decreased in 90% of dogs and is expected based on the
inhibition of phosphorus reabsorption by PTH in the kidneys.
▪There are only two differentials for a dog or cat with hypercalcemia and hypophosphatemia: primary
hyperparathyroidism and hypercalcemia of malignancy.
▪Total and ionized calcium concentrations are increased and are due to PTH stimulated bone resorption, renal
reabsorption of calcium, and indirectly from calcitrol
stimulated intestinal absorption of calcium.
Primary hyperparathyroidism
▪The total serum calcium is increased in 100% of dogs with primary hyperparathyroidism, yet ionized calcium is
increased in 90–95% of these dogs.
▪Despite marked hypercalcemia, less than 5% of primary
HPTH cases have azotemia. They rarely develop soft tissue mineralization because of concurrent hypophosphatemia.
▪The Ca × P product is typically less than 70.
▪The approach to the diagnosis involves use of screening
and confirmatory tests.
Secondary hyperparathyroidism
▪This is a chronic parathyroid response secondary to one of two disorders of calcium–phosphorus metabolism.
▪In secondary hyperparathyroidism, either renal disease or a calcium-phosphorus imbalance nutritional problem
initiates absolute or relative hypocalcemia and hyperphosphatemia.
▪Both nutritional and renal secondary hyperparathyroidism will produce bone lesions that range from mild osteolysis only detected radiographically to fractured bones to
enlarged bones from excess fibrous tissue deposition.
Secondary hyperparathyroidism
▪ Renal secondary hyperparathyroidism is usually easy to diagnose.
• It is associated with hypocalcemia,
hyperphosphatemia, and severe chronic renal disease with marked azotemia,
inability to concentrate urine, and nonregenerative anemia.
• Measurement of ionized calcium and or repeat measurements of total and ionized calcium may detect a nadir of
hypocalcemia, but there will be wide fluctuations if calcium is measured frequently.
Secondary hyperparathyroidism
▪Nutritional secondary hyperparathyroidism is a disease of carnivores, exotics (iguanas, etc.), and horses.
Ruminants are more likely to have osteoporosis or rickets (vitamin D or phosphorus deficiency).
• In the majority of cases, the serum concentration of calcium is within the reference interval and there is mild to moderate
hyperphosphatemia.
• It is associated with diets that either have insufficient calcium or too much phosphorus, or that have a calcium : phosphorus
imbalance, such that the ratio of calcium : phosphorus in the diet is no longer 2:1.
Secondary hyperparathyroidism
▪Fractional excretion of electrolytes, measurements of parathyroid hormone, and dietary evaluation with
measurements of calcium and phosphorus in feed are
usually the best ways to screen and confirm the diagnosis of nutritional secondary hyperparathyroidism.
▪Fractional excretion (Fx Exc) of electrolytes = (Serum Crea/Urine Crea) x (Urine Electr./Serum Electro) x 100
▪A Fx Exc Na of >1% indicates renal impairment.
Iatrogenic - thyroidectomy or removal of a parathyroid adenoma
▪Removal of the thyroid glands to treat hyperthyroidism in cats often results in complete or partial parathyroidectomy.
Therefore, hypocalcemia and hyperphosphatemia may ocur within 24–48 hours of the surgery, but can be delayed for up to one week. This also happens in dogs when a parathyroid tumor is excised.
▪Cats and dogs with iatrogenic hypocalcemia usually only
require treatment with intravenous or oral calcium and/or
vitamin D if clinical signs occur or if the total calcium is <7
mg/dL, ionized <1 mmol/L.
Your Questions?
Send to serkan.sayiner@neu.edu.tr
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