Thyroid gland 1 + 2 Flashcards
Where are the thyroid glands located?
Compromised of two distinct lobes on either side of
the trachea
Describe the endocrine cells in the thyroid glands
Follicular cells produce thyroid hormones
Parafollicular, medullary or C cells produce calcitonin
Thyroid hormone secretion is regulated by?
The hypothalamic-pituitary-thyroid axis
The thyroid gland secretes two major hormones, what are they?
Thyroxine and triiodothyronine, commonly called T4 and T3
What is required in the diet for thyroid hormone synthesis?
Iodine
Describe the hypothalamic pituitary thyroid axis and the production of hormones
- TSH secretion by the pituitary is modulated by thyroid hormone in a negative feedback regulatory mechanism
- TSH increases synthesis and secretion of T4 and T3 from the thyroid gland
- In the peripheral tissues T4 is converted into T3
- T4 and T3 have a negative feedback mechanism on the hypothalamus and pituitary gland
Name the major modulator of thyroid hormone concentration
Thyroid stimulating hormone
Describe the overall production of T3 and T4
The thyroid glands produce all circulating T4 but only up to 40% of circulating T3
Describe unbound and bound T3 and T4
> 99% of T4 and T3 are bound to plasma proteins (TBG)
Only unbound thyroid hormone is active (reservoir of bound thyroid hormone)
Is T3 or T4 being described:
rapid onset of action and more potent
T3
What are the functions of the thyroid hormones in the body
- Increase the metabolic rate and O2 consumption of most tissues
- Positive inotropic and chronotropic effects on the heart (Contractility and rate)
- Increase the number and affinity of β-adrenergic receptors and enhance the response to catecholamines
- Have catabolic effects on muscle and adipose tissue – i.e. will get rid of these tissues when the hormones are in excess
- Stimulate erythropoiesis and regulate cholesterol synthesis and degradation
Hypothyroidism is seen in which spp?
Dogs
What is hypothyroidism?
Clinical syndrome resulting from a deficiency of T3 and T4
Which dogs are predisposed to hypothyroidism
- Middle age to older dogs (tends to develop at an earlier age in breeds predisposed to lymphocytic thyroiditis)
- Predisposed breeds include English Cocker spaniels, golden retrievers and Doberman pinchshers – but any dog can get this condition
How is hypothyroidism categorised?
Depends on whether the underlying lesion lies within the thyroid gland (primary), pituitary gland (secondary) or hypothalamus (tertiary)
Also described as congenital or acquired, depending on the age at which it develops
Is congenital or acquired hypothyroidism most common?
Acquired
What are the causes of congenital hypothyroidism
- Caused by thyroid hypoplasia, aplasia or dyshormonogenesis
-> Disproportionate dwarfism in GSD - Fox and rat terriers
- Also may develop in pups born to dams fed an iodine-deficient diet, dams given various drugs, or puppies exposed to iodine deficiency or excesses when young
Name the 3 types of primary acquired hypothyroidism
Lymphocytic thyroiditis
Idiopathic atrophy
(Aggressive/extensive thyroid neoplasia)
Describe Lymphocytic thyroiditis?
Destructive autoimmune process characterized by multifocal or diffuse infiltration of the thyroid gland by lymphocytes, macrophages and plasma cells and progressive replacement by fibrous connective tissue
Describe idiopathic atrophy
Degenerative process with minimal inflammatory change and gradual replacement of thyroid tissue by adipose and connective tissue
Describe secondary/tertiary acquired hypothyroidism
Defect in the pituitary or hypothalamus (<5% of hypothyroid dogs) - E.g. head trauma
Describe the main clinical signs of hypothyroidism
- Metabolic and dermatological signs are seen most commonly
- Lethargy
- Weight gain
- Thinning/sparse hair coat. Loss of hair - On dorsal muzzle common or tail
- Exercise intollerence
List some other, less common signs of hypothyroidism
Cardiovascular, neuromuscular, reproductive, ophthalmic, and/or gastrointestinal signs
Hypothyroidism can present in a similar way to Cushings, how can they be differentiated?
A dog with hypothyroidism wont have PU/PD