What proportion of circulating triiodothyronine and thyroxine is bound to plasma transport proteins?

1. Less than one percent of circulating thyroid hormone remains unbound in blood. 2. That free fraction is the part able to cross the lipid bilayer and enter cells. 3. The remaining 99 percent is bound to thyroxine-binding globulins, albumin or other plasma proteins. 4. Binding packages the hormone and prevents its free diffusion into body cells. 5. When blood levels start to fall, bound hormone is released from those proteins and enters target cells. 6. Choosing 1 percent inverts free for bound, and the intermediate figures would leave far more free hormone than the gland actually allows. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 17 "The Endocrine System", section 17.4 The Thyroid Gland_