27.2 Thyroid and Antithyroid Drugs

Thyroid and Antithyroid Drugs

Thyroid and antithyroid drugs regulate hormone levels in patients with underactive or overactive thyroid conditions. Because thyroid hormones affect nearly every organ system (heart rate, metabolism, temperature regulation, energy), getting these medications right has a big impact on patient outcomes. Nurses need to understand the drug classes, their side effects, and the monitoring required to keep patients safe.

Key Features of Thyroid and Antithyroid Medications

Thyroid medications are used to treat hypothyroidism by replacing the hormones the thyroid gland isn't producing enough of.

• Levothyroxine (Synthroid, Levoxyl) is a synthetic form of thyroxine ($T_4$). It's the first-line treatment for hypothyroidism. $T_4$ is a longer-acting hormone that the body converts to the more active $T_3$ as needed, which makes dosing more stable and predictable.
• Liothyronine (Cytomel) is a synthetic form of triiodothyronine ($T_3$). It has a faster onset and shorter half-life than levothyroxine. It's sometimes used in combination with levothyroxine or alone when patients don't convert $T_4$ to $T_3$ well, but it's used less commonly because its levels fluctuate more throughout the day.

Antithyroid medications treat hyperthyroidism by blocking the production of excess thyroid hormones.

• Methimazole (Tapazole) inhibits the thyroid peroxidase enzyme, which reduces thyroid hormone synthesis. It's the preferred first-line antithyroid drug because it has a longer duration of action (once-daily dosing) and a lower risk of severe hepatotoxicity compared to PTU.
• Propylthiouracil (PTU) also inhibits thyroid peroxidase, but it has an additional mechanism: it blocks the peripheral conversion of $T_4$ to $T_3$. PTU is generally reserved for two situations: the first trimester of pregnancy (where methimazole carries a risk of birth defects) and thyroid storm, where that extra peripheral blocking action is beneficial.

Therapeutic Effects and Potential Side Effects of Thyroid Hormone Replacement Drugs

Therapeutic effects:

• Restores normal thyroid hormone levels, relieving symptoms of hypothyroidism such as fatigue, weight gain, cold intolerance, constipation, and dry skin
• Improves overall metabolic function, which affects heart rate, energy, mood, and cognitive function

Potential side effects:

• Overtreatment is the most common concern. If the dose is too high, the patient essentially develops iatrogenic hyperthyroidism: tachycardia, palpitations, tremors, anxiety, insomnia, heat intolerance, and unintended weight loss. This is why dosing starts low and is titrated gradually.

• Cardiovascular risks deserve special attention. In older adults or patients with underlying heart disease, excess thyroid hormone can trigger angina, arrhythmias (especially atrial fibrillation), or worsen heart failure.

• Decreased bone density can occur with long-term overtreatment, particularly in postmenopausal women, due to increased bone turnover.

• Drug interactions are common. Several medications reduce levothyroxine absorption:

  • Calcium and iron supplements
  • Cholestyramine and other bile acid sequestrants
  • Proton pump inhibitors and antacids Levothyroxine also potentiates the effects of warfarin, increasing bleeding risk. When a patient starts or adjusts thyroid replacement, their INR should be monitored closely.

• Allergic reactions are rare but can include rash, itching, or swelling.

Nursing Considerations When Administering Antithyroid Medications

1. Assess baseline and ongoing symptoms. Before starting therapy and at each visit, evaluate for signs of hyperthyroidism (tachycardia, tremors, heat intolerance, weight loss, anxiety) to gauge whether the medication is working.

2. Monitor thyroid function tests regularly. TSH and free $T_4$ levels guide dosage adjustments. Initially, labs are checked every 4–6 weeks until levels stabilize, then less frequently. The goal is to bring TSH into the normal range.

3. Watch for agranulocytosis. This is a rare but life-threatening drop in white blood cells that can occur with both methimazole and PTU. Teach patients to report fever, sore throat, or any signs of infection immediately. A CBC should be obtained if these symptoms develop. This side effect is most likely in the first 3 months of therapy.

4. Monitor for hepatotoxicity. PTU carries a higher risk, but both drugs can cause liver damage. Watch for jaundice, dark urine, abdominal pain, or unexplained fatigue. Baseline liver function tests should be obtained before starting therapy.

5. Reinforce medication adherence. Patients should not stop antithyroid medications abruptly without consulting their provider, as this can cause a rebound in hyperthyroid symptoms or precipitate thyroid storm.

6. Pregnancy considerations. If a patient of childbearing age is on methimazole and becomes pregnant, the provider will likely switch to PTU during the first trimester due to methimazole's teratogenic risk. After the first trimester, the provider may switch back to methimazole because of PTU's hepatotoxicity risk.

Patient Education Plan for Someone Starting Thyroid Hormone Therapy

What the medication does: Levothyroxine replaces the thyroid hormone your body isn't making enough of. It won't work overnight. Most patients start noticing improvement in symptoms within 2–4 weeks, but it can take 6–8 weeks to see the full effect of a given dose.

How to take it correctly:

1. Take it on an empty stomach, 30–60 minutes before breakfast (or at least 4 hours after your last meal). This maximizes absorption.
2. Take it at the same time every day for consistent levels.
3. Separate it from calcium supplements, iron supplements, multivitamins, and antacids by at least 4 hours, as these significantly reduce absorption.
4. Swallow with a full glass of water.

When to contact a provider:

• Signs of overtreatment: rapid heartbeat, palpitations, excessive sweating, tremors, unexplained weight loss, or feeling anxious or jittery
• Signs of allergic reaction: rash, itching, or swelling
• If you miss doses frequently or want to stop the medication, talk to your provider first rather than stopping on your own

Follow-up and monitoring:

• Expect blood work (TSH, free $T_4$) every 4–6 weeks initially until your dose is stabilized, then typically every 6–12 months
• Dosage adjustments are normal and don't mean something is wrong. Many factors (weight changes, pregnancy, aging, other new medications) can shift what dose you need.

Lifestyle support: A balanced diet, regular exercise, and stress management all support thyroid health. There's no special "thyroid diet" required, but patients should know that very high-fiber diets or large amounts of soy can modestly affect levothyroxine absorption, so consistency matters more than avoidance.