How Does Westhroid Work? Mechanism of Action Explained in Plain English

If you've been prescribed Westhroid, you might wonder how a tablet made from pig thyroid glands actually helps your body. The answer involves some fascinating biology—but it's actually quite intuitive once you understand what the thyroid does. Here's a plain-English breakdown.

What Does the Thyroid Gland Do?

Your thyroid is a butterfly-shaped gland at the base of your neck. Its main job is to produce hormones that regulate your body's metabolism—essentially, the rate at which your cells convert nutrients and oxygen into energy.

The thyroid produces two main hormones:

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[@portabletext/react] Unknown block type "span", specify a component for it in the `components.types` prop — the active form. T3 is the hormone that actually enters your cells and turns on gene expression, accelerating metabolism. It's about 4x more potent than T4.

When your thyroid doesn't produce enough of these hormones (hypothyroidism), your metabolism slows down. You feel tired, gain weight, get cold easily, lose hair, and experience brain fog—because your cells aren't getting the energy signals they need.

How Does Westhroid Replace These Hormones?

Westhroid contains dried and powdered thyroid tissue from pigs. Pig thyroid glands produce the same T3 and T4 hormones as the human thyroid, in very similar proportions. When you take a Westhroid tablet, your digestive system absorbs these hormones into your bloodstream—just as if your thyroid had produced them directly.

Each grain (65 mg) of Westhroid provides:

38 mcg of T4 (levothyroxine) — the storage hormone that gets converted to T3 throughout the day

9 mcg of T3 (liothyronine) — the active hormone that directly stimulates cellular metabolism

Why Do Some Patients Prefer Westhroid Over Levothyroxine?

Levothyroxine (Synthroid) only provides T4. The idea is that your body will convert the T4 to T3 as needed. And for many patients, this works well.

However, some patients don't convert T4 to T3 efficiently—a process called "peripheral conversion" that happens in organs like the liver, kidneys, and muscles. If this conversion is impaired (due to nutritional deficiencies, chronic illness, stress, or genetic variations), a patient may have normal T4 levels on paper but still feel hypothyroid because their cells aren't getting enough active T3.

Westhroid provides both T3 and T4 directly, which bypasses the conversion issue. For these patients, the direct T3 supply can make a significant clinical difference.

What Happens After You Take Westhroid?

Here's the step-by-step process:

You swallow the tablet on an empty stomach (for best absorption)

T3 and T4 are absorbed through the intestinal wall into the bloodstream — T3 peaks within 2–3 hours, T4 within 4–6 hours

T3 travels directly to cells throughout the body — it enters the cell nucleus and binds to thyroid hormone receptors, activating genes that control metabolism, heart rate, temperature, and more

T4 circulates in the bloodstream and is gradually converted to T3 by deiodinase enzymes in tissues — providing a steady baseline of thyroid hormone throughout the day

The feedback loop: as T3 and T4 levels rise, the pituitary gland reduces its output of TSH (thyroid-stimulating hormone), signaling that enough thyroid hormone is present

Why Does TSH Matter?

TSH (thyroid-stimulating hormone) is the master regulator. When thyroid hormone levels are low, the pituitary releases more TSH to push the thyroid to produce more. When levels are high, TSH drops. Your doctor uses TSH to monitor how well your Westhroid dose is working:

High TSH = dose too low (not enough thyroid hormone)

Low TSH = dose too high (too much thyroid hormone)

Normal TSH = dose is appropriate

Understanding how Westhroid works also explains why many of its side effects are signs of too much thyroid hormone — read our Westhroid side effects guide for a complete breakdown.

Having trouble finding Westhroid? medfinder can search pharmacies near you to find which ones have it in stock.