Thyroid Medications
Key Points
- Levothyroxine (Synthroid) is the first-line treatment for hypothyroidism — synthetic T4 with narrow therapeutic index requiring individualized dosing and TSH monitoring.
- Take levothyroxine in the morning, 30–60 minutes before food; avoid coffee, calcium, iron supplements, and soy within 4 hours — significantly impair absorption.
- Antithyroid agents (methimazole, propylthiouracil/PTU) are used for hyperthyroidism; PTU is preferred in first trimester of pregnancy and thyroid storm.
- All thyroid drugs require cardiac monitoring — thyroid hormones increase heart rate and contractility.
Pathophysiology
Hypothyroidism results from insufficient thyroid hormone (TH) production — causes include autoimmune disease (Hashimoto’s), radioiodine ablation, or iodine deficiency. Thyroid hormones (T3/T4) regulate basal metabolic rate, cardiac function, thermogenesis, and neurological development.
Hyperthyroidism (excess TH) causes increased metabolic rate, cardiac dysrhythmias, heat intolerance, and weight loss. Graves’ disease is the most common cause.
Drug Classification
| Drug Class | Examples | Indication |
|---|---|---|
| Thyroid hormone replacement | Levothyroxine (Synthroid, Levoxyl), liothyronine (Cytomel) | Hypothyroidism, myxedema coma, TSH suppression |
| Antithyroid agents (thioamides) | Methimazole (Tapazole), propylthiouracil (PTU) | Hyperthyroidism, thyroid storm |
| Iodine preparations | Potassium iodide (SSKI), Lugol solution | Thyroid storm, pre-thyroidectomy |
Nursing Assessment
NCLEX Focus
Monitor TSH levels — the primary lab for thyroid hormone replacement adequacy. Normal TSH: 0.4–4.0 mIU/L. Over-treatment causes cardiac arrhythmias and bone loss.
For hypothyroidism (levothyroxine):
- Assess baseline TSH, T4, heart rate, and blood pressure.
- Assess for concurrent medications that decrease levothyroxine absorption: calcium carbonate, iron supplements, antacids, cholestyramine, soy.
- Assess for cardiac disease — initiate at low dose in older adults and cardiac patients; titrate slowly.
- Assess for adrenal insufficiency before starting thyroid hormone — treating hypothyroidism without cortisol replacement can precipitate adrenal crisis.
For hyperthyroidism (antithyroid drugs):
- Assess CBC before initiation — methimazole and PTU can cause agranulocytosis.
- Monitor liver function — PTU carries hepatotoxicity risk; methimazole is preferred unless in first trimester.
Nursing Interventions
- Levothyroxine: Administer at same time each morning on empty stomach; wait 30–60 minutes before eating or drinking caffeinated beverages.
- Avoid administering with calcium, iron, antacids, or sucralfate — separate by at least 4 hours.
- Instruct patients not to switch between levothyroxine formulations/brands without provider guidance — bioavailability differences.
- Monitor for signs of overtreatment: tachycardia, palpitations, heat intolerance, weight loss, insomnia.
- For antithyroid agents: monitor for agranulocytosis (fever, sore throat, mouth sores) — hold drug and obtain CBC immediately.
- For potassium-iodide preparations (SSKI/Lugol), teach dose separation from other thyroid drugs by at least one hour and use dilution/straw techniques to reduce tooth staining.
- For radioactive-iodine therapy teaching, reinforce short-term radiation precautions (typically 1-3 days): avoid close contact, especially with pregnant people, avoid sharing utensils, flush toilet multiple times, and separate laundry per protocol.
Narrow Therapeutic Index
Levothyroxine dosing is highly individualized (1.6 mcg/kg/day standard; dose adjusted every 4–6 weeks based on TSH). Over-treatment (TSH <0.4) causes atrial fibrillation, bone mineral density loss, and cardiac complications. Under-treatment causes persistent hypothyroid symptoms.
Drug Interactions — Levothyroxine
Multiple medications impair levothyroxine absorption or alter metabolism: calcium, iron, antacids (separate by ≥4 hours); amiodarone (inhibits T4→T3 conversion); warfarin (thyroid hormone enhances warfarin effect — monitor INR); and soy-based foods.
Pharmacology
| Drug | Dose | Key Considerations |
|---|---|---|
| Levothyroxine (Synthroid) | 1.6 mcg/kg/day orally; titrate by 12.5–25 mcg every 4–6 weeks | Monitor TSH; take on empty stomach; consistent brand use |
| Liothyronine (Cytomel) | 25–75 mcg/day orally | Synthetic T3; faster onset; used for myxedema coma |
| Methimazole (Tapazole) | 10–60 mg/day in divided doses | Preferred antithyroid; avoid in 1st trimester pregnancy |
| Propylthiouracil (PTU) | 100–300 mg/day in divided doses | Preferred in thyroid storm and 1st trimester; hepatotoxicity risk |
Clinical Judgment Application
Clinical Scenario
A patient on levothyroxine 100 mcg/day starts a calcium supplement and reports increased fatigue and weight gain. Repeat TSH is elevated at 7.2 mIU/L.
- Recognize Cues: Elevated TSH = under-treated hypothyroidism; calcium supplement recently added.
- Analyze Cues: Calcium significantly impairs levothyroxine absorption when taken together.
- Prioritize Hypotheses: Drug-nutrient interaction causing reduced levothyroxine bioavailability.
- Generate Solutions: Educate patient to separate levothyroxine and calcium by ≥4 hours; provider may increase dose.
- Take Action: Educate on timing; notify provider of interaction.
- Evaluate Outcomes: Symptoms resolve; TSH returns to 0.4–4.0 range on repeat testing.
Related Concepts
- endocrine-system - Thyroid gland physiology and feedback regulation.
- heart-failure - Untreated hypothyroidism contributes to cardiac dysfunction.
- anticoagulants - Levothyroxine enhances warfarin effect; INR monitoring required.
- systematic-ecg-interpretation-and-dysrhythmia-triage - Hyperthyroidism causes atrial fibrillation; over-replacement carries same risk.
Self-Check
- What instructions must patients receive about taking levothyroxine to ensure proper absorption?
- What TSH range indicates adequate thyroid hormone replacement?
- When is PTU preferred over methimazole, and why?