Physiology of hypothyroidism Flashcards
What impact does hypothyroidism have on development?
- Lack of thyroid during fetal period and first few month after birth causes reduced development and maturation of brain cells in fetus and young
- Causes cretinism in humans
What causes cretinism?
Congenital lack of thyroid gland or iodine deficiency in mother
What impact does hypothyroidism have on growth?
- Growth retardation
- Smaller
- Shorter bones
- Delayed closure of physes
Describe the impacts of hypothyroidism on the metabolism
- Slows carbohydrate metabolism
- Slows fat metabolism
- Lowers basal metabolic rate to 50% of normal
- Results in weight gain
- Reduced muscle tone
- Bradycardia
- Nervous system= slower reflexes, lethargy and need more sleep
- Constipation
- Arrests hair growth- stuck at telogen. Resulting in alopecia or failure to regrow after clipping
What affects does thyroid hormone have on carbohydrate metabolism?
-Stimulates glucose metabolism: increases glucose uptake, insulin sensitivity, insulin secretion, glycolysis, gluconeogenesis
What affects does thyroid hormone have on the metabolism of fats?
- Thyroid hormone enhances fat metabolism
- Mobilises lipids from adipose stores
- Accelerates oxidation of lipids to produce energy (occurs within mitochondria- increase in size/ number of mitochondria)
What affects does thyroid hormone have on the basal metabolic rate?
- Increases BMR in all tissues EXCEPT brain, gonads, spleen
- Increases heat production
- Increases oxygen consumption
What induces weight loss?
-Levothyroxine
What are the normal affects of thyroid hormone on the cardiovascular system?
- Increases blood flow and cardiac output
- Increases heart rate
- Increases contractility
What affects does thyroid hormone have on the nervous system?
Aids development, enhances sympathetic nervous system and is needed for optimal nerve conduction
What affects does thyroid hormone normally have on the GI system?
- Increases appetite and feed intake
- Increases secretion of pancreatic enzymes
- Increases motility
What impact does thyroid hormone have on the integument?
-Initiates and maintains anagen phase
Where is most of the T4 and T3 produced?
All of T4 in the thyroid gland
Most of T3 in peripheral tissues
How does thyroid hormone move round the body?
Most circulating thyroid hormone is protein bound
Describe tertiary hypothyroidism
- At the level of the hypothalamus
- Lack of TRH
Describe secondary hypothyroidism
- At level of the pituitary
- Reduced secretion of TSH from pituitary
- Can be acquired (more common) or congenital (rare)
- Aquired causes= neoplasia, pituitary suppression (glucocorticoid administration), illness, malnutrition
- Congenital causes= cystic rathke’s pouch
Describe primary hypothyroidism
- Lack of functional thyroid tissue
- Most common form of hypothyroidism
- Can be acquired/ iatrogenic or congenital
- Most common= acquired: lymphocytic thyroiditis, isiopathic follicular atrophy
Which breeds are more prone to developing canine hypothyroidism?
-Doberman pinschers, golden retrievers, cocker spaniels, irish setter, terriers
What age does canine hypothyroidism normally occur?
Between ages 4-6
What are the 3 types of hypothyroidism and what causes them?
- Primary= lack of functional thyroid tissue
- Secondary= lack of TSH
- Tertiary= Lack of TRH
What are the clinical signs of canine hypothyroidism?
General appearance= Dullness Lethargy Exercise intolerance Obesity Cold intolerance (seek heat) 'Tragic expression' 'Rat tail' Bilaterally symmetrical alopecia Comedomes Bradychardia
What are the dermatological signs of canine hypothyroidism?
- Bilateral symmetrical alopecia
- Seborrhea, lichenification, comedones
- Recurrent infections e.g. otitis externa
Discuss the neurological abnormalities involved in canine hypothyroidism
-Segmental demylinisation and slow nerve conduction = knuckling, hearing impairment, slow gait
What is Myxedema?
-Excess mucopolysaccharides and hyaluronic acid in the dermis-> contributes to the tragic facial expression seen in canine hypothyroidism
