1.3 Dyslipidemia & Thyroid Disorders

Dyslipidemia & Thyroid Disorders

Cardiovascular risk reduction and endocrine balance are closely linked in primary care. Managing dyslipidemia through risk-directed statin therapy and diagnosing thyroid dysfunction through standard serological testing are crucial skills for primary care clinicians.

1. Dyslipidemia Management & ASCVD Risk Stratification

Pharmacological management of dyslipidemia focuses on lowering low-density lipoprotein cholesterol (LDL-C) to prevent atherosclerotic cardiovascular disease (ASCVD).

ASCVD Risk Scoring

For primary prevention (patients without established ASCVD), clinical decisions are guided by the 10-Year ASCVD Risk Calculator (using the Pooled Cohort Equations). This tool estimates the risk of a first cardiovascular event (nonfatal MI, coronary heart disease death, or stroke) over the next decade.

• Low Risk: < 5%
• Borderline Risk: 5% to < 7.5% (consider statin if risk-enhancing factors like family history of premature ASCVD are present)
• Intermediate Risk: 7.5% to < 20% (initiate moderate-intensity statin)
• High Risk: >= 20% (initiate high-intensity statin)

Primary vs. Secondary Prevention Strategies

1. Secondary Prevention: Patients with established ASCVD (history of MI, stroke, TIA, stable/unstable angina, coronary revascularization, or peripheral arterial disease) are at extremely high risk. Regardless of baseline lipid levels, these patients must be initiated on a high-intensity statin with a goal of achieving a >= 50% reduction in LDL-C and an absolute target LDL-C < 55 mg/dL or < 70 mg/dL depending on the specific international guideline.
2. Primary Prevention:
   • Severe Hypercholesterolemia (LDL-C >= 190 mg/dL): Initiate a high-intensity statin without performing risk scoring.
   • Diabetes Mellitus (Aged 40–75): Diabetic patients automatically qualify for at least a moderate-intensity statin regardless of their calculated ASCVD risk score. If their 10-year risk is >= 20% or multiple risk factors are present, high-intensity statin therapy is indicated.
   • General Population (Aged 40–75): Statin initiation is determined by the 10-year risk score (initiate moderate-intensity if risk is 7.5%–20%).

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2. Statin Intensity and Safety Profile

Statins (HMG-CoA reductase inhibitors) are classified by their ability to lower LDL-C:

Adverse Effects & Drug Interactions

• Statin-Associated Muscle Symptoms (SAMS): Ranges from simple myalgias (bilateral, proximal muscle pain/weakness without CK elevation) to myopathy (pain with CK elevation) and rhabdomyolysis (severe pain, creatine kinase [CK] > 10 times the upper limit of normal, brown urine containing myoglobin, and acute renal failure).
• Monitoring: Check baseline liver enzymes (ALT) prior to starting a statin. Routine serial monitoring of ALT or CK is not recommended unless the patient becomes symptomatic (e.g., muscle aches, dark urine, or signs of hepatotoxicity like jaundice).
• Contraindications: Gemfibrozil must never be co-administered with statins because it inhibits statin glucuronidation, raising statin serum levels and dramatically increasing the risk of rhabdomyolysis. If a fibrate is necessary in combination with a statin, fenofibrate is preferred.

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3. Thyroid Disorders: Diagnosis and Interpretation

Evaluation of thyroid function begins with measuring Thyroid Stimulating Hormone (TSH). If TSH is abnormal, a Free Thyroxine (FT4) level is obtained.

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4. Management of Hypothyroidism

• Pharmacotherapy: Levothyroxine (synthetic T4) is the treatment of choice.
• Dosing Guidelines:
  • Young, Healthy Adults: Full replacement dose of approximately 1.6 mcg/kg/day (based on ideal body weight).
  • Elderly Patients or Patients with Coronary Artery Disease (CAD): Initiate at a low dose of 25 mcg daily (or 12.5 mcg in frail patients) and titrate slowly. Levothyroxine increases cardiac output and myocardial oxygen demand. Rapid replacement can precipitate angina, myocardial infarction, or tachyarrhythmias (such as atrial fibrillation) in vulnerable patients.
• Monitoring: Check TSH 6–8 weeks after initiating therapy or adjusting the dose. Do not check earlier, as it takes several weeks for the pituitary-thyroid feedback loop to stabilize.

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5. Management of Hyperthyroidism

Etiologies

1. Graves' Disease: Autoimmune activation of the TSH receptor. Characterized by a diffuse, symmetric goiter, exophthalmos, and diffuse, homogenous uptake on a Radioactive Iodine Uptake (RAIU) scan.
2. Toxic Multinodular Goiter & Toxic Adenoma: Autonomously functioning nodules. RAIU shows focal area(s) of increased uptake with suppression of the surrounding gland.
3. Thyroiditis: Transient inflammation releasing preformed hormone. Characterized by a painful or painless thyroid and very low/absent RAIU because the gland is damaged and not actively synthesizing hormone.

Therapeutic Strategies

• Symptom Control: Beta-blockers (e.g., Propranolol) are initiated immediately to control hyperadrenergic symptoms (palpitations, tremors, anxiety, heat intolerance) while awaiting definitive therapy.
• Thionamides (Antithyroid Drugs):
  • Methimazole: First-line for most patients due to once-daily dosing and lower risk of hepatotoxicity. It is, however, teratogenic and contraindicated in the first trimester of pregnancy.
  • Propylthiouracil (PTU): Preferred in the first trimester of pregnancy (lower teratogenic profile) and in default scenario of thyroid storm (as it blocks the peripheral conversion of T4 to active T3, whereas methimazole does not).
• Life-Threatening Adverse Effect: Both thionamides can cause agranulocytosis (severe depletion of white blood cells). Patients must be instructed to immediately stop the drug and obtain a CBC with differential if they develop a fever, sore throat, or other signs of infection. PTU also carries a black-box warning for severe hepatotoxicity.
• Definitive Treatment: Radioactive Iodine (RAI) ablation or surgical thyroidectomy (preferred for large goiters, compressive symptoms, or suspected malignancy). Both options typically result in permanent hypothyroidism, requiring lifelong levothyroxine replacement.