6.5: Endocrine & Metabolic Therapeutics (Diabetes & Thyroid)

Endocrine & Metabolic Therapeutics in Australian Practice

Management of diabetes mellitus and thyroid disorders represents a significant portion of community and clinical pharmacy practice in Australia. Pharmacists must apply guidelines from the Australian Diabetes Society (ADS), the Royal Australian College of General Practitioners (RACGP), and the Australian Medicines Handbook (AMH) to optimize therapy and manage serious adverse events.

1. Diabetes Mellitus: Glycemic Targets & Monitoring

Glycemic management must be individualized to balance the benefits of microvascular and macrovascular risk reduction against the risks of hypoglycemia and treatment burden.

HbA1c Targets in Adults

• ≤ 6.5% (48 mmol/mol): Appropriate for patients with newly diagnosed type 2 diabetes, long life expectancy, no established cardiovascular disease, and low risk of hypoglycemia.
• ≤ 7.0% (53 mmol/mol): The general target for most adults with type 1 or type 2 diabetes.
• ≤ 8.0% (64 mmol/mol): Target for patients with a history of severe hypoglycemia, limited life expectancy, cognitive impairment, or established microvascular/macrovascular complications.
• Monitoring Frequency: Every 3 to 6 months.

Self-Monitoring of Blood Glucose (SMBG) Targets

For patients performing finger-prick monitoring, general targets are:

• Fasting / Pre-prandial: 4.0-7.0 mmol/L.
• Post-prandial (2 hours post-meal): 5.0-10.0 mmol/L (or 5.0-8.0 mmol/L if tight control is targeted).

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2. Pharmacological Selection in Type 2 Diabetes

The ADS/RACGP treatment algorithm outlines a stepwise escalation of pharmacotherapy.

First-Line Therapy: Metformin

Metformin remains the gold-standard first-line agent, working primarily by reducing hepatic gluconeogenesis and improving peripheral insulin sensitivity.

• Counselling: Titrate slowly (e.g., start 500 mg daily with dinner, increase by 500 mg weekly to 1000 mg twice daily) to minimize gastrointestinal adverse effects (nausea, abdominal bloating, diarrhea). Take with or immediately after meals.
• Renal Cut-offs:
  • Contraindicated if CrCl < 30 mL/min due to the risk of metformin-associated lactic acidosis (MALA).
  • Reduce maximum dose to 1000 mg daily if CrCl is 30-45 mL/min.
  • Review dose if CrCl falls below 60 mL/min.

Cardiovascular and Renal Protection Agents (Second-Line)

If a patient has established atherosclerotic cardiovascular disease (ASCVD), heart failure (HFrEF), or chronic kidney disease (CKD) with albuminuria, select an agent with proven cardiorenal benefit, independent of baseline HbA1c.

1. SGLT2 Inhibitors (e.g., Empagliflozin, Dapagliflozin)

• Mechanism: Inhibit sodium-glucose co-transporter 2 in the proximal renal tubule, promoting glycosuria.
• Benefits: Reduce heart failure hospitalizations and slow the progression of CKD.
• Side Effects: Genitourinary tract infections (thrush), urinary tract infections, increased risk of lower limb amputation (rare), and euglycemic diabetic ketoacidosis (DKA).
• Critical Counselling & Sick Day Rules:
  • Patients must withhold SGLT2 inhibitors during periods of acute illness, vomiting, diarrhea, fasting, or major surgery (from 3 days prior until normal oral intake resumes). This prevents euglycemic DKA (where blood glucose may be normal or only mildly elevated, masking ketoacidosis).
  • Maintain perineal hygiene to prevent mycotic infections.

2. GLP-1 Receptor Agonists (e.g., Semaglutide, Dulaglutide)

• Mechanism: Mimic glucagon-like peptide-1, enhancing glucose-dependent insulin secretion, suppressing glucagon secretion, and slowing gastric emptying.
• Benefits: Superior weight loss and significant reduction in major adverse cardiovascular events (MACE) in patients with established ASCVD.
• Side Effects: Nausea, vomiting, diarrhea, and injection site reactions.
• Precautions: Contraindicated in patients with a personal or family history of medullary thyroid carcinoma. Warn patients about the risk of acute pancreatitis (instruct them to seek immediate medical attention if they experience severe, persistent abdominal pain radiating to the back).

Other Antidiabetic Classes

• DPP-4 Inhibitors (e.g., Sitagliptin, Linagliptin): Prevent the breakdown of endogenous GLP-1. They are weight neutral and have a low risk of hypoglycemia.
  • Clinical Pearl: Linagliptin is the only DPP-4 inhibitor that does not require dose adjustment in renal impairment, as it is primarily excreted unchanged in the faeces.
  • Combination: Do not combine DPP-4 inhibitors with GLP-1 receptor agonists due to overlapping mechanisms of action.
• Sulfonylureas (e.g., Gliclazide, Glimepiride): Stimulate insulin secretion from pancreatic beta cells.
  • Risks: High risk of hypoglycemia and weight gain.
  • Elderly: Avoid long-acting sulfonylureas, particularly glibenclamide, in elderly patients and those with renal impairment, as active metabolites accumulate and cause prolonged, severe hypoglycemia. Gliclazide is preferred due to shorter half-life and inactive metabolites.
• Thiazolidinediones (e.g., Pioglitazone): Improve peripheral insulin sensitivity.
  • Risks: Fluid retention, weight gain, risk of bone fractures, and bladder cancer.
  • Contraindication: Absolute contraindication in patients with heart failure (NYHA Class I-IV) due to fluid retention.

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3. Thyroid Disorders: Hypothyroidism Management

Hypothyroidism is characterized by a deficient secretion of thyroid hormones, treated with synthetic levothyroxine (T4).

Dosing and Dosing Adjustments

• Standard Dosing: The replacement dose is approximately 1.6 mcg/kg/day.
• Elderly & Ischemic Heart Disease (IHD): Start with a very low dose (25 mcg daily) and titrate slowly (increase by 25 mcg every 4-6 weeks). Rapid correction of hypothyroidism in patients with IHD can precipitate angina, myocardial infarction, or tachyarrhythmias (atrial fibrillation).

Patient Counselling

• Administration: Take levothyroxine first thing in the morning on an empty stomach with a full glass of water, at least 30-60 minutes before breakfast, coffee, or other medications. Alternatively, it can be taken at bedtime (at least 3-4 hours after the last meal).
• Interactions: Separate levothyroxine by at least 4 hours from substances that impair its absorption, such as calcium carbonate, ferrous sulfate, antacids containing aluminium/magnesium, proton pump inhibitors, and colestyramine.
• Monitoring: Measure serum TSH levels 6-8 weeks after starting or altering the dose. Once stable, monitor TSH annually.

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4. Thyroid Disorders: Hyperthyroidism Management

Hyperthyroidism is characterized by excessive thyroid hormone synthesis. First-line pharmacotherapy utilizes antithyroid drugs (thionamides).

Pharmacological Selection and Pregnancy Guidelines

• Carbimazole: The preferred antithyroid drug in Australia due to once-daily dosing and a lower risk of hepatotoxicity.
• Propylthiouracil (PTU): Requires multiple daily doses.
• Pregnancy Considerations:
  • First Trimester: PTU is the drug of choice. Carbimazole is associated with congenital malformations, including aplasia cutis (congenital absence of skin, typically on the scalp) and choanal atresia.
  • Second and Third Trimesters: Switch the patient from PTU to carbimazole to minimize the risk of maternal hepatotoxicity, which is a rare but severe risk associated with PTU.

Life-Threatening Adverse Effect: Agranulocytosis

Agranulocytosis (profound neutropenia) is a rare (0.2-0.5%) but potentially fatal side effect of both carbimazole and PTU. It typically occurs within the first 3 months of initiating therapy.

• Patient Education (Symptom Monitoring): Pharmacists must counsel all patients to immediately stop taking their antithyroid medication and seek urgent medical attention if they develop symptoms of infection:
  • Fever
  • Sore throat
  • Mouth ulcers
  • Unexplained bruising or bleeding
• Action Protocol: If a patient presents with these symptoms, they must stop the drug immediately and have an urgent Full Blood Count (FBC) performed. The medication must not be restarted until agranulocytosis is ruled out (neutrophil count ≥ 1.5 x 10^9/L).

Hepatotoxicity Monitoring

• PTU carries a black box warning for severe liver injury and acute liver failure. Monitor liver function tests (LFTs) at baseline and if the patient develops signs of liver dysfunction (pruritus, jaundice, dark urine, right upper quadrant pain).