Levothyroxine
Levothyroxine (Synthroid) replaces the thyroid hormone T4 in people whose gland underproduces it. The product comes in seven strengths, from 25 mcg to 200 mcg, for Hypothyroidism and Thyroid-stimulating Hormone Suppression.
Key points
- Levothyroxine must not be used for weight loss; in people with normal thyroid function, doses within daily requirements are ineffective, and larger doses can cause serious or life-threatening toxicity.
- Levothyroxine sodium has a narrow therapeutic index, so both under- and over-treatment carry risks to heart, bone and metabolic function.
- For secondary or tertiary hypothyroidism, a TSH blood test is not a reliable way to judge whether the dose is right.
How does Levothyroxine work?
Levothyroxine is synthetic T4, the hormone your thyroid gland normally makes. Once absorbed, about 80% of it converts in your tissues into T3, the form that drives cellular activity. T3 enters the nucleus of your cells and binds to receptor proteins attached to DNA, switching on the genes that control how fast your body burns energy and builds proteins. An underactive thyroid slows the body down; replacement restores normal metabolic rate. Getting the dose right is central to Thyroid Health.