Levothyroxine
- Class: Hormone replacement
- Used for: an underactive thyroid
- Say it: lee-voh-thye-ROX-een
- Forms: tablet, capsule, injection
LEVOTHYROXINE LIFTS the underactive thyroid — and everything it had let sag rises with it.
It replaces the missing hormone, so metabolic rate, energy, bowel speed and heart rate all come back up. Lift it too fast in an older heart and you provoke angina or atrial fibrillation.
T4 is the storage hormone the body converts to active T3 as it needs it. That buffer is why one daily tablet gives a steady level, and why a dose change takes about six weeks to show.
Side effects
Common: palpitations, insomnia, heat intolerance
Serious: atrial fibrillation, bone loss
Typical dose
50–150 micrograms once daily
Micrograms, not milligrams — a dose written in milligrams would be a thousandfold overdose. Roughly 1.6 micrograms per kg in full replacement, adjusted in 25-microgram steps.
Check yourself
Why must Levothyroxine be taken on an empty stomach?
- Food, calcium and iron markedly reduce its absorption
- It irritates the stomach lining
- It only works when insulin levels are low
- Stomach acid destroys it chemically
Show answer
A. Food, calcium and iron markedly reduce its absorption
The problem is purely absorption. Taken with a calcium tablet, a large part of the dose never reaches the blood — which looks exactly like an inadequate dose on the TSH result.
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