💊 Levothyroxine

Levothyroxine is a synthetic preparation of thyroxine (T4), the primary hormone secreted by the thyroid gland. It is a thyroid hormone replacement therapy.

Levothyroxine replaces endogenous thyroxine, which is then deiodinated to triiodothyronine (T3) in peripheral tissues. T3 is the biologically active form of thyroid hormone, which binds to nuclear thyroid hormone receptors, regulating gene expression and influencing metabolism, growth, and development.

The primary indication is for the treatment of hypothyroidism, regardless of its cause (e.g., Hashimoto's thyroiditis, post-thyroidectomy, congenital hypothyroidism). It is also used as thyroid-stimulating hormone (TSH) suppression therapy in certain thyroid cancers.

Untreated adrenal insufficiency is an absolute contraindication, as thyroid hormone replacement can precipitate an adrenal crisis. Uncorrected thyrotoxicosis and acute myocardial infarction are also contraindications. Hypersensitivity to the active substance or excipients is rare but possible.

Adverse effects are typically dose-related and reflect symptoms of hyperthyroidism, such as palpitations, tachycardia, arrhythmias (especially in elderly patients or those with cardiac disease), angina, tremors, insomnia, and weight loss. Osteoporosis can occur with long-term overtreatment.

Thyroid function tests (TSH and free T4) are essential for monitoring treatment efficacy and adjusting the dose. TSH is the most sensitive marker for adequate replacement. Monitoring should occur initially at 6-8 week intervals until stable, then annually or if symptoms change.

Always check for adrenal insufficiency before initiating, especially in patients with pituitary disease. Counsel patients on the importance of consistent daily dosing, typically on an empty stomach, and avoiding interactions with other medications (e.g., iron, calcium, PPIs). Explain that it's a lifelong medication and regular monitoring is needed.

High-yield for its role in hypothyroidism and the importance of TSH as the primary monitoring parameter. Remember that it should be taken consistently, usually in the morning on an empty stomach, and separate from other medications. Be aware of the symptoms of over- and under-replacement. Untreated adrenal insufficiency is a key contraindication.

SBAs often test the monitoring parameters (TSH and free T4, not T3) and the interpretation of results (e.g., high TSH, low T4 indicates under-replacement). Questions may also focus on drug interactions (e.g., iron, calcium, PPIs) or the management of subclinical hypothyroidism. Expect scenarios involving patients with symptoms of under- or over-replacement.