Thyroid Disorders: Hypothyroidism, Hyperthyroidism & Subclinical Disease

Thyroid Disorders: Hypothyroidism, Hyperthyroidism & Subclinical Disease

Thyroid dysfunction represents one of the most frequent endocrine disorders encountered in adult and geriatric primary care. Thyroid hormones—Thyroxine (T4) and Triiodothyronine (T3)—play an essential physiological role in regulating basal metabolic rate, cardiac inotropy/chronotropy, systemic vascular resistance, lipid metabolism, neuromuscular activity, and thermogenesis. Clinical management requires precise understanding and interpretation of the Hypothalamic-Pituitary-Thyroid (HPT) axis negative feedback loop.

Diagnostic Interpretation Matrix

The single best initial screening test for thyroid dysfunction is the serum Thyroid-Stimulating Hormone (TSH) level. If the TSH is abnormal, a Free T4 level is measured to differentiate overt disease from subclinical states. Serum Free T3 is primarily useful when hyperthyroidism is suspected but Free T4 is normal.

Primary Hypothyroidism

Primary hypothyroidism results from intrinsic functional failure of the thyroid gland. In iodine-sufficient regions, the predominant etiology is Hashimoto's thyroiditis (chronic autoimmune lymphocytic thyroiditis), an autoimmune disorder characterized by cell-mediated destruction of thyroid tissue accompanied by elevated serum anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies.

Clinical Presentation & Physical Examination

Clinical onset is typically insidious. Symptoms include generalized fatigue, cold intolerance, unexplained weight gain despite decreased appetite, chronic constipation, dry/scaly skin, diffuse hair thinning, hoarseness, memory impairment, depression, and menorrhagia. Physical examination features bradycardia, mild diastolic hypertension, periorbital puffiness, non-pitting edema (myxedema), coarse brittle hair, macroglossia, and classic delayed relaxation of deep tendon reflexes (hung-up reflexes, best demonstrated at the Achilles tendon).

Levothyroxine (L-T4) Pharmacotherapy & Dosing Strategies

Synthetic Levothyroxine is the gold-standard monotherapy for hypothyroidism, acting as a prohormone converted peripherally to active T3.

1. Young, Healthy Adults (< 60 years without CAD):

   • Full replacement dosage is 1.6 mcg/kg/day based on ideal body weight (IBW).
   • For a 70 kg patient, a typical starting dose is 100 to 112 mcg daily.

2. Older Adults (>= 65 years) or Patients with Known CAD:

   • "Start Low and Go Slow" Rule: Initiate therapy conservatively at 12.5 to 25 mcg daily.
   • Clinical Rationale: Rapid reintroduction of thyroid hormone increases myocardial oxygen consumption and cardiac workload, which can precipitate severe angina, coronary vasospasm, myocardial infarction, or ventricular arrhythmias.
   • Titrate by 12.5 to 25 mcg increments every 6 to 8 weeks based on repeat serum TSH levels.

Administration Rules & Drug Interactions

Levothyroxine must be taken consistently in the morning with a full glass of water on an empty stomach, at least 30 to 60 minutes before breakfast, or at bedtime at least 3 hours after the last meal.

• Malabsorption Interference: Patients must separate Levothyroxine administration by at least 4 hours from calcium carbonate, ferrous sulfate, aluminum/magnesium antacids, PPIs, sucralfate, cholestyramine, and high-fiber/soy supplements.

Monitoring Protocol & Myxedema Coma

Recheck serum TSH 6 to 8 weeks after initiating therapy or adjusting dosage. Once euthyroid status is established (target TSH 0.5–4.0 mIU/L; 1.0–3.0 mIU/L in younger adults), monitor TSH annually. Myxedema coma is a life-threatening complication occurring in elderly patients presenting with severe hypothermia (< 95°F), profound bradycardia, altered mental status/coma, hypoventilation, and hyponatremia. Treatment: Immediate IV bolus Levothyroxine (200–400 mcg), IV hydrocortisone (100 mg q8h to prevent precipitating adrenal crisis if coexisting Addison's disease is present), and passive rewarming.

Subclinical Hypothyroidism

Subclinical hypothyroidism is defined as an elevated serum TSH (> 4.5 mIU/L) with a normal Free T4 level.

• Indications for Levothyroxine Treatment (25–50 mcg/day):
  1. TSH is > 10 mIU/L (prevents progression to overt disease, diastolic dysfunction, and lipid abnormalities).
  2. Patient is pregnant or planning pregnancy (essential for early fetal brain development).
  3. Patient has positive anti-TPO antibodies accompanied by symptomatic goiter or fatigue.
• If TSH is 4.5–9.9 mIU/L in an asymptomatic, non-pregnant adult without positive antibodies, monitor serum TSH and Free T4 every 6 to 12 months.

Hyperthyroidism & Thyrotoxicosis

Hyperthyroidism is characterized by excessive thyroid hormone synthesis and secretion. The leading etiology in younger adults is Graves' disease (autoimmune stimulation of TSH receptors by Thyroid-Stimulating Immunoglobulins [TSI] / TRAb). In older adults, Toxic Multinodular Goiter and Toxic Adenoma predominate.

Clinical Presentation & Physical Signs

Symptoms include heat intolerance, unintentional weight loss despite hyperphagia, palpitations, fine resting tremors, emotional lability, insomnia, hyperdefecation, and oligomenorrhea. Signs include sinus tachycardia/atrial fibrillation, warm moist skin, hyperreflexia, and smooth enlarged goiter. Graves-Specific Features: Exophthalmos (proptosis, periorbital edema) and pretibial myxedema (infiltrative dermopathy).

Management Triad

1. Beta-Blockers (Propranolol 10–40 mg QID or Atenolol): Immediate symptom control for adrenergic manifestations (tachycardia, tremor). Propranolol at high doses also blocks peripheral T4-to-T3 conversion.
2. Antithyroid Thionamide Pharmacotherapy:
   • Methimazole (MMI): Preferred agent for non-pregnant patients due to rapid onset, once-daily dosing, and lower hepatotoxicity risk. Dose: 10–30 mg daily.
   • Propylthiouracil (PTU): Preferred drug of choice ONLY during the 1st trimester of pregnancy (prevents MMI embryopathy) and in Thyroid Storm (inhibits peripheral T4 conversion). Black Box Warning: Fatal fulminant hepatic necrosis.
   • Agranulocytosis: Rare but severe bone marrow suppression. Patients MUST stop the drug immediately and obtain an urgent CBC if they develop a sudden fever or sore throat.
3. Definitive Therapies: Radioactive Iodine (RAI-131) ablation (contraindicated in pregnancy, lactation, and severe active orbitopathy; results in permanent post-ablation hypothyroidism) or subtotal/total thyroidectomy.

Evaluation of Thyroid Nodules

1. Measure serum TSH.
2. If TSH is low, perform a Radionuclide Thyroid Scan (I-123): "Hot" nodules are hyperfunctioning (rarely malignant); "cold" nodules require ultrasound.
3. If TSH is normal/elevated, order a Dedicated Thyroid Ultrasound.
4. Fine-Needle Aspiration (FNA) Biopsy: Indicated for nodules >= 1 cm with high-risk ultrasound features (microcalcifications, solid hypoechoic, irregular margins) or nodules >= 1.5–2 cm without high-risk features.