The Thyroid Gland

Question 1

What us the Hypothalamic pituitary thyroid axis?

Answer 1

• Hypothalamus releases TRH that acts on the pituitary gland
• Pituitary gland releases TSH that acts on the thyroid gland
• Thyroid gland releases T3 + T4 that goes into peripheral tissues
• T4 turns into T3 and acts as negative feedback on the hypothalamus and the pituitary gland

Question 2

What does the thyroid gland do?

Answer 2

• Produces all circulating T4 but only up to 40% of T3
• > 99% of T4 + T3 are bound to plasma proteins (TBG)
• Only unbound thyroid hormone is active
• T3 has more rapid onset of action and is more potent than T4

Question 3

What do thyroid hormones do?

Answer 3

• Increase metabolic rate + O2 consumption of most tissues
• Have positive inotropic and chronotropic effects on the heart
• Increase the number and affinity of β-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 4

What is congenital hypothyroidism?

Answer 4

• Caused by thyroid hypoplasia, aplasia or dyshormonogenesis
• Disproportionate dwarfism
• Fox and rat terriers (autosomal recessive)
• Isolated TSH/TRH deficiency reported in a family of giant schnauzers and in a young boxer
• Feature of panhypopituitarism in GSD

Question 5

What are different acquired hypothyroidism?

Answer 5

• Primary =
• Lymphocytic thyroiditis
• Idiopathic atrophy
• (Aggressive / extensive thyroid neoplasia)
• Secondary / Tertiary =
• Defect in the pituitary or hypothalamus (uncommon)

Question 6

What are clinical signs of hypothyroidism?

Answer 6

• Metabolic signs
• Lethargy
• Obesity / weight gain
• Exercise intolerance
• Cold intolerance
• Dermatological abnormalities = alopecia, poor coat, skin hyperpigmentation, pyoderma, seborrhoea

Question 7

How is hypothyroidism diagnosed?

Answer 7

• Haematology + Serum Biochemistry (only suggestive) =
  -Mild normocytic normochromic non-regenerative anaemia
  -Hypercholestrolaemia
  -hypertriglyceridaemia
  -mild increase in CK
  -mild increase in ALP + ALT
  -increased fructosamine
• Thyroid hormone testing - Total T4, Free T4, Total T3, cTSH

Question 8

How would you treat hypothyroidism? + Monitor

Answer 8

• Levothyroxine - care in cardiac patients
• monitor = measure tT4 4-6hrs post treatment
• metabolic signs resolve within a week, dermatological signs within 2-3months + neuro signs within 6 months

Question 9

What are complications of hypothyroidism?

Answer 9

• Neurological =
• Peripheral neuropathy
• Generalised myopathy
• Megaoesophagus
• CNS
• Hyperlipidaemia, athersclerolsis, Gall Bladder mucocele
• Myxoedema coma

Question 10

What does v high cholestrol suggests in a dog?

Answer 10

• Hypothyroidism

Question 11

What is the different hypothyroidism cats get?

Answer 11

• Spontaneous (naturally occurring) = acquired primary hypothyroidism
• Secondary hypothyroidism - due to head trauma
• Congenital
• Iatrogenic (MOST COMMON)

Question 12

What can cause iatrogenic hypothyroidism in cats

Answer 12

• overdose of anti-thyroid drug administration
• bilateral thyroidectomy
• radioactive iodine
• hypophysectomy

Question 13

What animals tend to get hyperthyroidism?

Answer 13

• Old cats

Question 14

What are most cases of hyperthyroidism in cats due to?

Answer 14

• Nodular adenomatous hyperplasia

Question 15

What are clinical signs of hyperthyroidism in Cats?

Answer 15

• PROGRESSIVE
• Weight loss (despite polyphagia)
• Vomiting
• Hyperactivity
• PU/PD
• Diarrhoea
• Tachycardia, heart murmur, cardiac failure
• Systemic hypertension
• Skin + Hair coat changes

Question 16

How can hyperthyroidism be diagnosed?

Answer 16

• Haematology + Serum biochemistry (suggestive) =
  -Mild-to-moderate erythrocytosis and macrocytosis (increased oxygen demand)
  -Increased Heinz bodies
  -Increased mean platelet size
  -Leukocytosis
  -Increased ALP, ALT, AST, LDH
  -Hypokalaemia, hyperphosphataemia
  -Decreased fructosamine
• Thyroid hormones = Total T4, Free T4, T4 suppression test
• Scintigraphy

Question 17

How do different drugs affect total T4, free T4 + TSH?

Answer 17

• Most drugs decrease tT4, fT4 + TSH
• phenobarbital + sulphonamides = increase TSH

Question 18

How is hyperthyroidism treated?

Answer 18

• Medical management
• Surgery - thyroidectomy - isn’t curative (50% of cats will get hyperthyroidism again)
• Radioiodine - treatment of choice
• Other therapies =
  -percutaneous ethanol injection
  -hyperthermia - US-guided heat ablation
  -iodine-restricted diet

Question 19

What are anti-thyroid drugs?

Answer 19

• Methimazole - inhibits synthesis of thyroid hormone
• Carbimazole - pro-drug converted to methimazole in-situ

Question 20

What are adverse reactions of anti-thyroid hormones?

Answer 20

• Anorexia
• Lethargy
• Vomiting
• Pruritus
• Excoriation
• Blood dyscrasias
• Thrombocytopenia
• Agranulocytosis
• Hepatopathy
• Acquired myasthenia gravis

Question 21

What are complications of thyroidectomy surgery?

Answer 21

• Iatrogenic hypoparathyroidism
• Laryngeal paralysis
• Horner’s syndrome - interruption of nerve supply to one half of the face
• Recurrent laryngeal nerve damage resulting in voice change
• Transient / permanent hypothyroidism

Question 22

Why do cats with hyperthyroidism + CKD have normal creatinine?

Answer 22

• Muscle loss
• Glomerular hyperfiltration of creatinine

Question 23

What is a thyroid storm / thyrotoxicosis?

Answer 23

• Condition of acute thyrotoxicosis in which the patient’s metabolic, thermoregulatory, and cardiovascular mechanisms are overwhelmed by excessive circulating levels of thyroid hormone

Question 24

What can cause hyperthyroidism / thyrotoxicosis in dogs?

Answer 24

• Functional, malignant thyroid tumour excreting thyroid hormone
• Iatrogenic thyrotoxicosis secondary to over-supplementation (most common) / accidental ingestion
• Dogs receiving raw food diet containing thyroid tissue