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Levothyroxine’s long half-life supports once-daily dosing, but its absorption can be reduced by food and several medicines. Dose adjustment therefore depends on appropriate administration and follow-up monitoring. Liothyronine acts more rapidly and is more potent, but its greater cardiac risk limits its use; liotrix and desiccated thyroid offer no established advantage over T4 alone in this source. The lesson also covers dose considerations in older adults, ischemic heart disease, and pregnancy, along with adverse effects, contraindications, and clinically relevant interactions. It concludes with special considerations such as changes in TBG, malabsorption, product switching, storage, and emergency management of overdose.