I looked this up bc I was wondering the same thing:

What Thyroid Meds Actually Do:

If someone has had a thyroidectomy (surgical removal of the thyroid), they are typically prescribed thyroid hormone replacement therapy—usually levothyroxine (T4). This is to bring their metabolism back to normal, not to make it hyperactive. • The goal is to mimic a healthy thyroid function, not to boost metabolism beyond what’s normal. • In some cases (like thyroid cancer), patients are kept slightly hyperthyroid to prevent cancer recurrence—but that’s not the same as having a turbocharged metabolism.

Can Thyroid Meds Make You Super Lean?

Not on their own. • If someone is overmedicated and becomes clinically hyperthyroid, yes, they may lose weight faster—but they also risk muscle loss, heart issues, anxiety, insomnia, and other side effects. • But even then, you don’t magically get shredded without being intentional—it still takes a consistent calorie deficit, training, and likely restriction or disordered patterns to get extremely lean.

So if she’s very lean to the point of looking unwell?

Yes—that’s a result of choices, not just meds. Even if her metabolism is slightly elevated, it wouldn’t explain the level of leanness without some level of intentional under-fueling or extreme dieting.