Episode 16 - Bonus Episode! Endocrinopathies I - Hyper and Hypothyroidism

1.The GP hook

Skinny, starey-coated cats and chunky, slow dogs walk into consult… here’s how we steer thyroid cases from “I think it’s endocrine” to confident diagnosis and calm, contextualised care.

Skinny, starey‑coated cats and chunky, slow dogs walk into consult… here’s how we steer thyroid cases from “I think it’s endocrine” to confident diagnosis and calm, contextualised care.

We cover the practical bits you’ll use tomorrow: what a hyperthyroid cat usually looks like, the first tests to run, how to explain the kidney‑unmasking problem, the real‑world pros and cons of medical vs diet vs radioactive iodine vs surgery, plus the simple, systematic approach to canine hypothyroidism and how to avoid over‑interpreting single numbers.

2.Thyroid basics — fast and useful

• Two thyroid lobes sit either side of the trachea; ectopic thyroid tissue can exist anywhere from just caudal to the larynx down into the thorax.
• HPT axis in a sentence: TRH → TSH → thyroid makes mainly T4 (a prohormone); tissues convert T4 to active T3.
• Clinical shorthand: too much thyroid = higher metabolic rate (hungry, lose weight, often tachycardic); too little = slower metabolism (weight gain, lethargy, dermatological change).

3.Feline hyperthyroidism — classic presentation and pitfalls

Typical cat you’ll see in practice:

Exam pearls

• Older cat with progressive weight loss despite an increased appetite; often more vocal and restless.
• Coat changes: “starey”, greasy or dull; sometimes poor grooming.
• Mild polyuria/polydipsia can occur; tachycardia, murmurs, gallops or arrhythmias may be present.
• Important differential trio for an older cat that’s eating more but losing weight: hyperthyroidism, diabetes mellitus, chronic enteropathy. CKD, hypertension and neoplasia are common comorbidities — don’t get tunnel vision.
• Try to palpate a thyroid “slip” by gently extending the neck and running thumb and finger down either side of the trachea. Some clinicians feel goitres frequently; others rarely — it can be present or absent in true disease.
• Record heart rate, rhythm (look for gallop or arrhythmia), hydration and body condition.

4.First‑line tests in the skinny, shouty cat

Eating lots + losing weight in an older cat? Your top three differentials: hyperthyroidism, diabetes and chronic enteropathy. Many of these cats also have CKD and/or hypertension in the background.

• Total T4 is the standard in‑house screening test. A clearly high result in a compatible cat is usually enough to proceed.
• If T4 is high‑normal or borderline but the cat screams thyroid clinically, consider repeating the sample or sending a free T4 (equilibrium dialysis) for extra sensitivity in early cases — interpret free T4 with caution in sick cats.
• Run a broad biochemistry (kidney, liver, glucose, proteins) and haematology in older cats to map comorbidities and look for non‑regenerative anaemia or inflammatory changes. B12/folate are useful if enteropathy is on the list.
• Measure blood pressure — hypertension is common and changes risk/management (retina, CNS, kidneys, heart). Beware white‑coat effects, but a clearly high systolic needs action.
• If you hear a gallop or arrhythmia, consider NT‑proBNP or a focused echo where available, particularly if you’re planning immunosuppressives, steroids or anaesthesia.

5.Medical therapy (methimazole/thiamazole/carbimazole; liquid, tablet, transdermal)

• Pros: widely available, reversible, helpful to stabilise while you assess kidneys/heart, multiple formulations to suit cat and owner.
• Watch‑outs: handle carefully (risk to owners with transdermal preparations); some cats develop facial pruritus after starting; occasional blood dyscrasias occur so haematology on early rechecks is sensible. Discuss safe handling and realistic pill delivery plans with owners.

6.Iodine‑restricted diet

• Feasible only for strictly indoor, highly compliant households — no other foods, treats or hunting. Real‑world adherence is the main barrier.

7.Radioactive iodine (RAI)

• One‑off definitive treatment for many cats; treats ectopic tissue too. Great option where referral, owner buy‑in and temporary home precautions are acceptable. Not suitable for every cat or owner.

8.Thyroidectomy

Practical owner conversation

• Less common now. Consider anaesthetic risk in older cats, risk to parathyroids (hypocalcaemia) and the possibility of missed ectopic tissue. Case selection and careful counselling are essential.
• Explain up front that treating the thyroid can reveal underlying CKD (hyperthyroidism can increase GFR and mask azotaemia). Make a recheck plan before starting therapy so owners don’t mistake an unmasked azotaemia for harm you caused.

9.Monitoring the treated hyperthyroid cat

Don’t skip the pre‑treatment owner chat about “kidneys after thyroid.” Book and document the recheck.

• Early recheck: many clinicians reassess clinical signs within 2–4 weeks for weight gain and check T4, haematology and renal parameters to detect dyscrasias and unmasked CKD.
• Data sheets commonly recommend an intensive early monitoring schedule followed by periodic checks; in practice tailor frequency to the cat’s stability and owner factors. If you sample less frequently than manufacturer guidance, document your rationale.
• Reassess blood pressure and heart status if signs persist or before procedures.

10.A quick note — feline hypothyroidism

• Genuine hypothyroidism in cats is rare and most commonly iatrogenic after treating hyperthyroidism. Presentation is not the same as classic canine hypothyroidism; only investigate when the signs fit.

11.Canine hypothyroidism — the slow burn

Typical signalment and signs:

Differential with Cushing’s

• Middle‑aged, medium–large dogs that have gained weight despite owner efforts; lethargy, exercise intolerance.
• Skin and coat changes are often central: symmetrical trunk/tail alopecia, hyperpigmentation, poor coat quality and hair epilation. “Tragic” myxoedematous facial expression and sometimes bradycardia.
• Neurological signs can occur but are less frequent in routine GP.
• Both can cause weight gain and symmetrical alopecia. Cushing’s commonly has marked polyuria/polydipsia and increased AP on biochemistry; hypothyroid dogs usually do not have marked PUPD and present more slowly.

12.Diagnosing canine hypothyroidism — avoid the single‑number trap

Sighthound caveat

• Don’t diagnose on a single low total T4 alone. Non‑thyroidal illness (euthyroid sick syndrome) commonly lowers T4.
• The classic supportive pattern is low total T4 with an elevated TSH. If clinical suspicion is high but TSH is normal, consider a fuller panel including thyroid autoantibodies — antibodies can affect T4 measurement and indicate underlying thyroiditis.
• Weigh the clinical picture heavily (dermatological changes, body condition) before starting lifelong supplementation.
• Sighthounds often have lower baseline T4 values — interpret in breed context and consider TSH/antibodies if suspicion persists.

13.Treatment and monitoring in dogs

Recognising over‑supplementation

• Levothyroxine is the standard treatment. Start, monitor and adjust systematically rather than reflexively.
• Monitoring: you can check either a trough (pre‑dose) or a peak (a few hours after dosing) T4 depending on your question:
• Peak sampling checks that the dose reaches a high‑normal concentration.
• Trough sampling assesses the steady‑state baseline.
• If the dog is clinically well, routine TSH testing isn’t required at every visit — interpret T4 timing and the clinical picture together.
• Adjust dose slowly and reassess clinical signs and appropriate blood tests.
• Signs of iatrogenic hyperthyroidism: weight loss with good appetite, excitability, panting, tachycardia, PUPD and GI upset. If these appear, recheck dosing and adherence first.

14.Practical in‑clinic tips that change outcomes

• Weigh every suspect thyroid cat and reweigh at each visit — weight trend is your fastest truth‑meter.
• Add a quick blood pressure to any older‑cat weight‑loss bundle.
• For hypothyroid‑suspect dogs, photograph coat patches at baseline — owners love seeing objective improvement on repeat visits.
• When owners ring for “a thyroid blood test,” decide whether you need a peak or trough before booking. A 30‑second vet callback prevents wasted visits and misinterpreted results.

15.Take‑home points

Thyroid cases reward a calm, broad first screen, a clear plan for kidneys, blood pressure and the heart, and an upfront owner conversation. Get those right and the endocrine win is satisfying and straightforward.

• Hyperthyroid cat essentials: older, ravenous yet losing weight, coat change, often tachycardic. Run total T4 with a broad screen and check blood pressure; consider free T4 if early/borderline but interpret in context.
• Warn owners that treating hyperthyroidism can unmask CKD and plan rechecks.
• Medical therapy is first‑line for many cats; radioactive iodine is a definitive option for suitable patients; iodine‑restricted diet requires strict exclusivity; surgery is selective.
• Hypothyroid dog essentials: gradual weight gain without marked PUPD, lethargy and symmetric dermatological changes. Don’t rely on a single low T4 — aim for low T4 + elevated TSH and use antibodies when needed.
• Levothyroxine works well if monitored correctly — choose peak vs trough checks intentionally and adjust slowly.