Internal Medicine: Endocrinology: Thyroid

Question 1

1. Where is the thyroid found in cats and dogs?
2. What are the two types of endocrine cells?
3. What is required for hormone synthesis?

Answer 1

1. Comprised of two distinct lobes on either side of the trachea
2. Follicular cells- thyroid, Parafollicular or C cells- calcitonin
3. Adequate dietary iodine is required

Question 2

1. Describe the functional unit (follicle) of the thyroid
2. What hormones do the thyroids secrete and what are their function?
3. What produces calcitonin and what is its function?

Answer 2

1. A sphere of cells with a lumen containing a clear proteinaceous colloid- containing primarily thyroglobulin- a large glycoprotein dimer that serves as reservoir for thyroid. Parafollicular C cells lie in the interstitium between follicles
2. T3 and T4- increase metabolic rate
3. Thyroid C cell- reduced plasma calcium concnetration by binding to an inhibiting osteoblasts- stopping resorption- calcitonin and parathyroid decrease renal tubular reabsorption of phosphorus

Question 3

How is thyroid secretion regulated?

Answer 3

• Hypothalamic- pituitary- thyroid axis
• Low plasma concentrations of thyroid hormones stimulates secretion of thyrotropin-releasing hormone (TRH)- by the hypothalamus and TSH by pituitary thyrotrophs
• Allmost all circulating T3/4 is bound to transthyrectin (protein bound)

Question 4

How does thyroid enter and act on cells?

Answer 4

• T3 and T4 enter cells by transporter proteins
• T3 enters faster and more rapid onset and more potent
• Thyroid binds to receptors in the nuclei
• Influences the expression of genes coding for regulatory enzymes
• Thyroid hormone is also believed to have some non-genomic effects

Question 5

What are the functions of thyroid hormones?

Answer 5

• Increase the metabolic rate- O2 consumption of most tissues
• Have positive inotropic and chronotropic effects on the heart
• Increase the number and affinity of beta-adrenergic receptors and enhance the response to catecholamines
• Have catabolic effects on muscle and adipose tissue
• Stimulate erythropoiesis and regulate cholesterol synthesis and degradation

Question 6

1. What is hypothyroidism?
2. What can be the different causes?
3. What are the predisposed breeds?

Answer 6

1. Clinical syndrome resulting from a deficiency of T3/4
2. Any abnormality in the hypothalamic-pituitary-thyroid axis- primary, secondary or tertiary (thyroid, pituitary, hypothalamus)- congenital or acquired
3. English cocker spaniels, golden retrievers, dobermans

Question 7

1. What causes congenital hypothyroidism?
2. How does it often clinically present?

Answer 7

1. Thyroid hypoplasia, aplasia or dyshormonogenesis
2. Disproportionate dwarfism

Question 8

What are the most common causes of primary and secondary/tertiary (acquired) hypothyroidism?

Answer 8

Primary- most common- destructive
* Lymphocytic thyroiditis
* Idiopathic atrophy
* Aggressive/extensive thyroid neoplasia

Secondary/tertiary
* Defect in the pituitary or hypothalamus

Question 9

What are the clinical signs of hypothyroidism?
How can congenital hypothyroidism appear?

Answer 9

Relate to metabolic and dermatological abnormalities
* Lethargy
* Hair thinning or alopecia
* Dry/poor quality coat
* Obesity/weight gain
* Excercise intolerance

Congenital
* Evident at 8 weeks of age
* Disproportionately wide skills
* Macroglossia
* Delayed dental eruption
* Square trunk and short limbs
* Possibly goitre

Question 10

How is hypothyroidims in dogs diagnosed?

Answer 10

Thyroid hormone required for diagnosis
* Measurment of total/free T3/4 and TSH
* Antibody measurment (TgAA, T4AA, T3AA)
* Dynamic function tests

Haematology and biochem will not diagnose hypothyroidism
* Mild normocytic normochromic non-regenerative anaemia
* Hypercholsteraemia

Question 11

What are the pros and cons of total thyroxine (TT4) test?

Answer 11

• Cheap and readily available
• Sensitive (low false negative)
• Not specific (high false positives)

TT4 is decreases in many non-thyroidal illnesses, and by many drugs
Some dogs- greyhounds and sighthounds have low normal values- use tt3

Question 12

What are the pros cons and considerations of free thyroxine (FT4)?

Answer 12

• Measured by equilibirum dialysis
• Concentration significantly increases if stored at room temp for prolonged periods
• Usually normal in animals with low tT4 due to non-thyroidal illness
• Less affected by the presence of T4 autoantibodies
• Good specifcity (low false false positives)
• Low sensitivity (high false negatives)

Question 13

What are the problesm of TSH test?

Answer 13

• High in hypothyroid dogs- can be normal
• Sometimes high with non-thyroidal illness, use of sulphonamides

Always measure in combination with tT4

Question 14

What are the pros and cons of autoantibodies tests of thyroid hormones?

Answer 14

• Provide evidence of active inflammation/immune reaction
• Not all dogs who test positive to TGAA become clinically hypothyroid
• Cause abberent test results- falsely increased tT4,tT3
• Autoantibodies can affect results

Question 15

Describe the process for diagnosing hypothyroidism

Answer 15

Question 16

What drugs can induce hypothyroidism?

Answer 16

Sulphonamides

Thyroid hormone concentrations normalise once drug therapy is withdrawn- 6 weeks

Question 17

How is hypothyroidism treated in dogs?

Answer 17

Hypothyroid dogs require thyroid hormone replacement therapy for life- synthetic T4 is treatment of choice
* Levothyroxine
* Care in cardiac patients
* Monitor tT4- 4-6 hours post treatment
* Metabolis signs should resolve within the first few weeks
* Dermatological- 2-3 months

Question 18

What are complications of hypothyroidism?

Answer 18

Neurological
* Peripheral neuropathy
* Generalised myopathy
* Megaoesophagus
* Laryngeal paralysis
* CNS

Hyperlipidaemia, atherosclerosis, GB, mucocoele
Myxoedema coma

Question 19

What are the three different causes of hypothyroidism in cats?
What test is the most sensitive for diagnosis?

Answer 19

• Spontaneous- primary- rare- typical clinical signs uncommon- goiture common sign
• Secondary- due to head trauma described
• Iatrogenic- most common
  Overdose of anti-thyroid admin
  Bilateral thyroidectomy
  Radioactive iodine
  Hypophysectomy

Canine TSH most sensitive test for diagnosis

Question 20

What age of cats have an increased chance of hyperthyroidism?
What cats have a decreased risk?

Answer 20

• Older cats- mean age 13-14, 5% over 9yo
• No breed or sex predisposition
• Decreased risk- persian, siamese, burmese, tonkinese, abyssininan, BSH

Question 21

1. What is the pathophyiology of most hyperthyroidism in cats?
2. What may be the underlying cause?
3. What are the clinical signs?

Answer 21

1. Nodular adenomatous hyperplasia
2. Nutrition (canned food), environmental, genetic
3. Weight loss, PUPD, hyperactivity, GI signs, skin and hair coat changes

Question 22

How is hyperthyroidism in cats diagnosed?

Answer 22

Haematology
* Mild to moderate erythrocytosis and macrocytosis
* increased heinz bodies
* Leukocytosis
* Increased ALP, ALT, AST, LDH
* Hypokalaemia, hyperphosphataemia
* Decreased fructosamine

Question 22

How is hyperthyroidism in cats treated?

Answer 22

• Medical therapy
• Surgery
• Radiodine
• Others

Question 23

What anti-thyroid drugs can be used to treat hyperthyroidism?
What is important for owners with use of these drugs?

Answer 23

Thiamazole
* Inhibits synthesis of thyroid hormone at several levels
* Check tT4 after 2 weeks then every 3 months
* Oral or transdermal

Carbimazole
* Inhibits synthesis of thyroid hormone at several levels
* Pro-drug
* Slow release form is licensed

• Owner compliance is needed- side effects occur often within the first 3 months including life threatening ones- thrombocytopenia, hepatopathy, agranulocytosis.
• If any of these reactions occur, the anti-thyroid drug must be stopped
• Trial in animals with suspected renal disease- can have normal creatinine due to muscle loss

Question 24

What are the pros of thyroidectomy surgery? What are the potential complications?

Answer 24

Pros * Effective * Available Complications * Iatrogenic hypoparathyroidism * Laryngeal paralysis * Horner's syndrome * Recurrent laryngeal nerve damage resulting in voice change * Transient or permanent hypothyroidism

Question 25

What is the treatment of choice for hyperthyroidism?

Answer 25

Radioiodine * Brachytherapy technique * Radioactive iodine concentrated in cells where there is upregulation of thyroid production * Beta particles travel about 2mm in tissue * Treats all hyperfunctioning tissue * Curative in >95%

Question 25

What is the treatment of choice for hyperthyroidism?

Answer 25

Radioiodine * Brachytherapy technique * Radioactive iodine concentrated in cells where there is upregulation of thyroid production * Beta particles travel about 2mm in tissue * Treats all hyperfunctioning tissue * Curative in >95%

Question 26

What other therapies can be used for feline hyperthyroidism?

Answer 26

Percutaneous ethanol injection * Only solitary nodules * Side effects due to leakage of ethanol Hyperthermia * Ultrasound guided heat ablation * Transient responses * All cases recurred within 18 months Diet * Iodine restricted diet

Question 27

What is SHIM-RAD

Answer 27

Severe hyperthyroidism Huge tumour size Intrathoracic tumour Multifocal disease pattern (3 or more nodules) Resistance to Antithyroidal Drug Some cats fail to respond to treatment- benign adenoma can transform to malignant carcinoma cells

Question 28

How can CKD be affected by hyperthyroidism?

Answer 28

* Successful treatment of hyperthyroidism has the potential to unmask pre-existing CKD * The associated changes in renal funciton are usually mild * Renal function typically stabilises within 6 months of treatment

Question 29

What is thyroid storm?

Answer 29

Thyroid storm is a condition of acute thyrotoxicosis in which the patients metabolic, thermoregultatory and cardiovasculat mechanisms are overwhelmed by excessive circulating levels of thyroid hormone

Question 29

What is thyroid storm?

Answer 29

Thyroid storm is a condition of acute thyrotoxicosis in which the patients metabolic, thermoregultatory and cardiovasculat mechanisms are overwhelmed by excessive circulating levels of thyroid hormone

Question 30

What are the most common causes of hyperthyroidism in dogs?

Answer 30

Functional, malignant thyroid tumour- 10-30% Iatrogenic thyrotoxicosis secondary to oversupplementation- most common Seen in dogs recieveing raw food diet containing thyroid tissue