A 2016 research study by the Endocrine Society showed that the thyroid hormone T3 controls and regulates the release of insulin, suggesting that low thyroid function could raise the risk of developing type 2 diabetes, especially in people with pre-diabetes.
This should really not be surprising at all as T3 is the active thyroid hormone. T3 is the only real thyroid hormone and it alone is able to bind to the thyroid receptors in the nucleus of every single cell. T3 enables all our cells to work at the correct rate and perform their proper function. Every major function in the body is regulated by the T3 thyroid hormone so having low free T3 levels is going to impact people in many different ways.
This is yet another reason for ensuring that FT3 is tested. Testing TSH or even TSH and FT4 will not reveal whether FT3 is adequate or not. Someone can have perfectly ‘normal looking’ TSH and FT4, yet still have low FT3. Someone can also have other issues that stops the proper function of even good levels of FT3. So, testing other things is also important. Cortisol for instance has to be at a healthy level and not high or low. High reverse T3 (rT3) can be an indicator that T3 is not as effective as it should be. See my The Thyroid Patient’s Manual book for more information on what additional tests to run and how to assess the results.
Here is a link to the article:
Also this article that suggests that a low Free T3 level increases mortality rates in diabetic patients: