Thyroid and Parathyroid Flashcards
Hypothalamic- Pituitary- Thyroid Axis
Hypothalamus: TRH
Pituitary: TSH
Thyroid: T4, T3 (with negative feedback ^^)
Thyroid Hormone
Thyroglobin synthesized by smooth endoplasmic reticulum
Ingested iodine converted to iodine in GI tract
Iodine
Daily requirement (higher in cats)
Thyroid hormones only iodinated organic compounds in the body
Hypothyroidism
In dogs and horses
CS: Lethargy, mental depression, weakness, non pruritic hair loss
Primary hypothyroidism
99% spontaneous disease
Cells lost due to idiopathic atrophy or lymphatic thyroiditis
Secondary hypothyroidism
Deficiency in TSH
Congenital pituitary malformation
Levothyroxine
Hypothyroidism tx
T4 supplementation (treats symptoms)
Start slow, then ↑
Efficacy 1-3 months of dosing
Therapeutic failure of Levothyroxine
Improper dose of frequency
Owners compliance
Problems with absorption, metabolism, or excretion
Outdated
Incorrect dx
Hyperthyroidism
Most common in geriatric cats
CS: Thyrotoxicosis (excess T4 and T3), WL, ravenous appetite, hyperactivity, PU/PD
Cardiac CS of Hyperthyroidism
Tachycardia, PVC, Gallop rhythms
Hyperthyroidism tx
Definitive therapy (thyroidectomy, radioactive iodine)
Medical management (methimazole)
I 131 (iodine therapy)
A cure given SQ*, PO, IV
Euthyroidism occurs 1-3 weeks of admin
Tx in cats (ectopic thyroid tumors)
MOA of I131
Actively taken up and concentrated by thyroid tissue
Beta particles target abnormal tissues
Disadvantage of I131 therapy
Available in specialty referral centers
Radiation safety precautions must be taken
Safe to go home in 4 days
Retreatment may be necessary
Advantages of I131
Least invasive
No toxicity
Survival time 25 months
Methimazole
Reverse thyrotoxicosis and maintains euthyroidism
T4 concentrations in the lower half of reference range
MOA of METHIMAZOLE
Blocks synthesis of thyroid hormones by preventing iodine and peroxidase from their normal interactions with thyroglobulin to form T3 and T4 (not secreted)
Methimazole characteristics
Does not affect tumor size
CS will recur with discontinuation
Used before sx to stabilize patient
Side effects of methimazole
Serum antinuclear abs
No CS
Disappears after drug discontinued
Thyroid and drug interactions
Glucocorticoids
Anticonvulsants
NSAIDs (salicylates and phenybutazone)
Antimicrobials (trimethoprim/ sulfa)
Radiocontrast agents
Parathyroid
Normal calcium homeostasis
Hypoparathyroid= hypocalcemia
Hyperparathyroid= hypercalcemia
Normal calcium homeostasis
Reabsorption of calcium and phosphorus in bone and distal renal tubular cells
Vit. D mediated absorption of calcium from intestinal tract
Hypocalcemia
↑ PTH → ↑ Ca resorption from urine → ↑ Ca and Phosphorus mobilization from bone → ↑ Vit. D synthesis
Hypoparathyroidism in dogs
Destruction of parathyroid glands by disease, trauma or sx removal
