Why does fasting lower your thyroid hormone T3 a little bit?

Sometimes, going on a fast can lower a thyroid hormone called T3. This is the active form of the thyroid hormone. 

The pituitary communicates to the thyroid through a hormone called the thyroid stimulating hormone. One of the main ways hypothyroidism is diagnosed is by looking at the thyroid stimulating hormone. When you’re fasting and doing the healthy keto diet, you won’t see a change in the thyroid stimulating hormone. So, you are not going to have a true hypothyroid condition. 

The thyroid releases T4, which is the inactive form of the thyroid hormone. To become activated, it has to work through the liver and into the kidney. It converts from T4 to T3 (the active form). There is also something called reverse T3, which is the inactive version of T3. What determines the rate and ratio of this conversion of T4 to T3 or reverse T3 is the metabolic need. 

When you do a fast, you may see a slight decrease in T3. But, you will see an increase in the reverse T3. You’re body is becoming more efficient with its fuel, so the need for the active form of the thyroid hormone is going to be slightly less. The body will then put some of that hormone into a reserve until you need it. 

Situations where you could see higher amounts of reverse T3:

1. When you’re doing fasting

2. When you’re recovering from:

• A stroke 

• An injury 

• Overtraining 

• An illness 

• A surgery 

3. When you’ve just lost a good amount of weight