Levothyroxine / Synthroid

A patient takes levothyroxine with breakfast and a calcium supplement — and wonders why they still feel exhausted. The drug is working against two absorption killers most students overlook.

Levothyroxine is a synthetic T4 that replaces deficient thyroid hormone in hypothyroidism (most commonly Hashimoto's thyroiditis and post-thyroidectomy). T4 is a prohormone — the body converts it to active T3 peripherally, providing steady, physiologic hormone levels. This is why levothyroxine is dosed once daily despite a long half-life of 6–7 days. Therapeutic response takes 4–6 weeks; TSH is the primary monitoring lab, checked every 6–8 weeks after initiation or dose change. The target TSH for most adults is 0.5–4.0 mIU/L. Absorption is the critical nursing issue: the drug must be taken on an empty stomach, 30–60 minutes before breakfast, with a full glass of water. Calcium, iron, antacids, and soy products bind levothyroxine in the gut and dramatically reduce absorption — separate these by at least 4 hours. Dose adjustments are small (12.5–25 mcg increments) because overreplacement mimics hyperthyroidism: tachycardia, palpitations, tremor, heat intolerance, weight loss, insomnia. In older adults or patients with cardiac disease, start low and titrate slowly — excess thyroid hormone increases myocardial oxygen demand and can precipitate angina or dysrhythmias.

Key Distinctions

Don't confuse levothyroxine overreplacement signs (tachycardia, tremor, weight loss) with undertreated hypothyroidism (bradycardia, fatigue, weight gain) — they are mirror images. Students mix up T4 and T3 drugs: levothyroxine is T4 (slow, stable, first-line); liothyronine is T3 (rapid, rarely used alone). A rising TSH means the dose is too LOW — TSH and thyroid hormone move inversely.

Clinical Pearl

Think of TSH as a thermostat calling for heat: high TSH means the body is cold (hypothyroid) and needs MORE levothyroxine, not less.