Thyroid Drugs

endocrine gland and organ abnormalities

• dysfunction originating in the peripheral endocrine gland (primary disorders)

• pituitary gland

  • Hyperfunction: Hyperplasia or neoplasia

  • Hypofunction: autoimmune disorders, tumors, infections, vascular disorders, toxins

• abnormal tissue responses to hormones

  • blockage of conversion from secretion to active form

• treatment

  • hypofunction = replacement of peripheral endocrine hormone

  • hyperfunction = radiation therapy, surgery, drug to suppress hormone production

Thyroid

• Iodine-dependent gland

• triiodothyronine (T3) and thyroxine (T4)

  • regulate the body’s metabolism by stimulating cells in the body to produce proteins

  • increases cellular metabolism

• calcitonin

• physiological responses

  • tachycardia

  • tissue growth and development

  • thermoregulation

  • energy consumption

• PE, patient history, and blood chemistry tests

normal thyroid gland function

• Hypothalamus » releases Thyrotropin-Releasing Hormone (TRH)

• TRH stimulates nearby pituitary gland

• Pituitary gland is stimulated to release Thyroid-stimulating Hormone (TSH)

• Pituitary TSH stimulates the thyroid gland to produce triiodothyronin (T3) or thyroxin (T4)

thyroid gland control

• T3 and T4 are released into the blood and taken up by cells in the body to increase the body’s metabolic rate

• T3 is the biologically active form of thyroid hormone (even though produced less than T4)

• T4 is converted by enzymes in the body tissues to T3 (removing an iodine converts T4 → T3)

• T4 acts as a circulating precursor reservoir of hormone to be converted to T3 as needed

• T3 and T4 provides negative feedback

Hypothyroidism

• low T4, high TSH

  • weight gain, bradycardia, lethargy, bilateral alopecia, dull/brittle hair, thin skin

• treatment depends to T3 and T4 levels in the blood

  • free T4 level (fT4) » circulating in blood, not bonded to protein

    • will be low cause gland not responding

  • thyroid stimulating hormone (TSH)

    • pituitary keeps producing TSH which will be elevated cause keeps producing cause no thyroid hormones produced

• synthetic hormone supplementation with regular hormone level monitoring

Levothyroxine

MOA:

• synthetic T4

  • preferred over synthetic T3 (liothyronine)

  • has longer half life so needs to be dosed less frequency

Use:

• hypothyroidism - treatment of choice

Forms:

• Soloxine; Thyro-Tabs

Side Effects:

• Rare

Notes:

• therapeutic monitoring - make sure do not over supplement and cause hyperthyroidism

Hyperthyroidism

• excessive TT4 or FT4

• commonly caused by benign hyperplasia (adenoma on thyroid gland)

  • weight loss, polyphagia, tachycardia, polyuria/polydipsia

• treatment options:

  • thyroidectomy, radioiodine therapy, anti-thyroid drug (ex: methimazole), iodine-limiting prescription diet

• choice based upon:

  • cost

  • client ability/willingness to administer daily medication life long

  • anesthetic/surgical risk

Methimazole

MOA:

• interferes with incorporation of iodine into precursors of T3 and T4

Use:

• feline hyperthyroidism

Forms:

• transdermal, oral liquid, or tablet

• Felimazole

• Tapazole

Side Effects:

• v+, anorexia, facial irritation

Notes:

• Carbimazole » another drug that is converted to Methimazole

• therapeutic monitoring - make sure don’t over suppress and cause hypothyroidism

• monitor renal values - hyperthyroidism causes hypertension and improves renal excretion and once controlled pressure normalizes and renal perfusion decrease to normal and can “unmask” renal disease

Radioactive Iodine

MOA:

• beta emission destroys over-productive thyroid tissue

Use:

• Feline Hyperthyroidism

Forms:

• Radioactive iodine (I-131); SQ, IV

Notes:

• Cat housed in special facility since will excrete radiation in feces, urine

  • after 3 days - 3 weeks cat is released

• Limited number of facilities that perform procedure

Dietary Food

MOA:

• prevent formation of T3/T4 by limiting iodine needed to create the hormones

• clinically proven nutrition to restore thyroid health

• improves thyroid health in 3 weeks

• complete daily nutrition

Nursing Info

• because iodine intake from other food sources - treats, another pet’s food, etc - can compromise the effectiveness of low-iodine nutrition, it’s critical that only y/d if fed exclusively