Library · Physiology
How hormones travel through the blood
Make it clearer
Some hormones have a logistics problem: they are lipophilic, meaning they mix easily with fat and do not dissolve in water. And blood is, essentially, water. It is the same problem as oil in a glass of water.
The solution is to travel with a companion. They latch onto transport proteins that do move comfortably through the blood, and that is how they travel through the body. Not all of them need this: steroid hormones, built from cholesterol, are among the lipophilic ones, while peptide hormones, assembled from amino acids, dissolve in water without help.
Thyroid hormone shows the detail that matters: it acts when it is free, not while it is latched on. And since most of it circulates latched on, if those transport proteins rise or fall for any reason, the total level the lab measures swings widely.
And here is what keeps that from being alarming: that swing in the total does not change the amount of free hormone or the rate at which the body uses it up. What changes is the number measured, not the hormone that is actually doing the work.
In brief
Lipophilic hormones do not dissolve in water, so they travel through the bloodstream bound to transport proteins.
Steroid hormones, made from cholesterol, are lipophilic; peptide hormones, made from amino acids, are water-soluble.
Thyroid hormone acts when it is free, and since most of it circulates bound, a change in those proteins shifts the total.
The messenger rides strapped to its escort and acts only once it lets go.
That change does not alter the free concentration or the rate at which the hormone is metabolized.
Sources (3)
- OpenStax, Rice UniversityAnatomy and Physiology 2e, 17.2 HormonesPublished
- National Center for Biotechnology Information, StatPearls PublishingBiochemistry, Hormones, StatPearlsPublished
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Endocrine Hormones, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
Updated