Hyper and hypothyroidism

• What is the Thyroid Gland?

  • Endocrine gland on both lateral aspect of trachea in the ventral neck
• What is the Thyroid Gland?

  • Main roles
  • Synthesize, release, and regulate thyroid hormones
  • Controls metabolic rate
• What is the Thyroid Gland?

  • Main roles
  • Growth and development of CNS and long bones
  • Also important for nerve conduction velocity
  • Impacts release of sex hormones
  • Maintains skin integrity
• What does it look like?

  • Has thin fibrous capsule around gland
• What does it look like?

  • Thyroid follicles
  • Area that synthesizes and stores thyroid hormone
  • Colloid- protein rich fluid containing thyroid hormones produced by the thyroid follicular epithelium
  • Lined by single layer of thyroid follicular epithelium
• What does thyroid Gland look like?

  • C cells- thyroid medullary cells
  • Fainter colored cells in middle
  • Calcitonin- a hormone that regulates calcium/phosphate levels within the body
• Thyroid Hormone

  • Called thyroxine T4 and triiodothyronine T3
  • T4 not as active → turns into T3
  • Bound by thyroxine-binding globulin
  • Very little is circulating freely in blood, but free is active hormone
• Thyroid Hormone

  Regulation:

  1. hypothalamus produces thyrotropin-releasing hormone (TRH)
  2. stimulates pituitary gland to make thyroid-stimulating hormone (TSH)
  3. stimulates glands to produce T4 and T3
• Thyroid Hormone

  • Iodine essential for hormone synthesis
• Hypothyroidism in Dogs

  • Primary: ~95% of cases are due to destruction of thyroid gland
• Hypothyroidism in Dogs

  • Most common causes of destruction
  • lymphocytic thyroiditis
  • Characterized by a diffuse infiltration of the gland by lymphocytes, plasma cells, and macrophages
  • Results in progressive destruction of follicles and secondary fibrosis
• Hypothyroidism in Dogs

  • Most common causes of destruction
  • Idiopathic atrophy of the thyroid gland
  • Characterized by loss of thyroid parenchyma and replacement by adipose tissue
• Hypothyroidism in Dogs

  • Secondary: destruction of pituitary thyrotrophs by an expanding, space-occupying tumor
  • Rare but can see with neoplasia in thyroid and congenital secondary due to dwarfism
• Hypothyroidism in Cats

  • Iatrogenic most common type
  • Develops after treatment for hyperthyroidism with radioiodine, surgical thyroidectomy, or use of an antithyroid drug
• Hypothyroidism in Cats

  • Can also have congenital or juvenile- onset
  • Causes of congenital hypothyroidism: intrathyroidal defects in thyroid hormone synthesis, an inability of the thyroid gland to respond to TSH, and thyroid improper development
• Hypothyroidism in Cats

  • Overall a very rare condition in cats
• Hypothyroidism- Risk Factors

  • Breeds predisposed: Golden Retriever, Doberman Pinscher, Irish Setter, Miniature Schnauzer, Dachshund, Cocker Spaniel, Airedale Terrier
• Hypothyroidism- Risk Factors

  • Females 2.5x more likely than males
  • Spayed females have higher risk than intact
• Hypothyroidism- Risk Factors

  • More common in dogs 4-10yrs of age
• Hypothyroidism- Clinical Signs

  • Lethargy/depression
  • Weight gain with no additional calorie intake
  • Bilateral alopecia
  • “Rat tail” alopecia
• Hypothyroidism- Clinical Signs

  • Hyperpigmentation to the skin or changes in fur texture
  • Thickened skin +/- “puffy” appearance to face (myxedema)
  • Cold intolerance
  • Recurrent skin infections
  • Infertility
• Hypothyroidism- Physical exam

  • BAR-QAR
  • Significant alopecia +/- dry, flaky skin
  • Sometimes have yeast odor due to secondary infections
• Hypothyroidism- Physical exam

  • Significantly overweight
  • Hyperpigmentation of the skin +/- complete changes to the hair
  • Also called acanthosis nigricans
• Hypothyroidism- Diagnostics

  • Recommended to do CBC/chem to rule out other diseases
  • ~66-75% will have hypercholesterolemia
  • 25-40% will have mild nonregenerative anemia
• Hypothyroidism- Diagnostics

  • Usually start with a total T4 → if borderline, will strongly recommend a full panel
  • Will include TT4, free T4, T3, and +/- TSH
• Hypothyroidism- Diagnostics

  • Other tests include thyroid imaging, TSH stimulation tests, biopsy, and measuring thyroid hormone antibody concentration
• T4, T3, TSH Bloodwork

  • Total T4 most common test used for diagnosis
  • Normal range: 1 to 4 μg/dl
  • If between 1-1.5 then need to look at clinical signs to decide if further testing is neede
• T4, T3, TSH Bloodwork

  • More conclusive panel includes TT4, free T4, T3, and TSH
  • There are several different panels with combos of these
• T4, T3, TSH Bloodwork

  • TSH stimulation tests are very expensive and difficult to find a lab who will analyze the data
• T4, T3, TSH Bloodwork

  • *IMPORTANT NOTE* Animal needs to be looked at as a whole picture
• T4, T3, TSH Bloodwork

  • Euthyroid sick- condition in which serum levels of thyroid hormones are low in patients who have nonthyroidal systemic illness but who are actually euthyroid
• Hypothyroidism - Treatment/ maintenance

  • Life-long oral medication
  • Levothyroxine- 0.01-0.02mg/kg/day
  • Can be given SID but most vets will split it and give BID for better control
• Hypothyroidism - Treatment/ maintenance

  • Will monitor T4 levels 4-8 weeks after start treatment
  • Adjust if needed and then recheck again in 4-8 weeks
  • If dose ok, recheck every 6 months to ensure no further changes needed
• Hypothyroidism - Outcome

  • Prognosis is good
  • Many dogs, if managed well, will live a full life with a good quality of life (QOL)
• Hyperthyroidism

  • Overproduction and secretion of T4 and T3 into the bloodstream
  • Causes increase in metabolic rate
• Hyperthyroidism

  • Most commonly caused by functional thyroid adenoma, or adenomatous hyperplasia
  • ~70% will have both glands enlarged
  • Thyroid carcinoma is the primary reason for this in dogs, but incredibly rare in cats (1-2%)
• How Does Hyperthyroidism Cause Murmursand Hypertension?

  • Thyroid hormone is involved in regulation of cardiovascular system
  • In sick animals, hormone is doing this without signaling from other pathways that it is needed
  • In normal animal, helps with regular needs of increased contractility, HR, and cardiac output
• How Does Hyperthyroidism Cause Murmurs and Hypertension?

  1. Increase contractility by affecting Ca2+ reabsorption
  2. increases cardiac
  3. output further exasperated by chronotropic effects of T3 on SNS stimulation
  4. increased pressure from contractions increases blood pressure
  5. thickens ventricular muscles and reduces lumen
  6. stretches atrial wall due to volume increase
  7. leads to hypertrophic cardiomyopathy
• Hyperthyroidism- Risk factors

  • Middle aged to older cats (average of 13yrs, range from 4-20yrs)
  • Also believe females at higher risk
• Hyperthyroidism- Risk factors

  • Theories (still debated) that non-pure bred cats are at higher risk