Essentials: How to Control Your Metabolism by Thyroid & Growth Hormone

Andrew Huberman, who introduces himself as a professor of neurobiology and ophthalmology at Stanford School of Medicine, opens this Huberman Lab Essentials episode by naming two hormone systems, thyroid hormone and growth hormone along with their related pathways. He describes them as arguably the two systems in the body most significant for setting overall metabolism. He says the podcast is separate from his teaching and research roles at Stanford, and he describes it as part of an effort to bring zero-cost information about science and science-related tools to the general public.

He defines metabolism broadly. It is not necessarily eating. It is the use of energy in the body's cells for growing tissue, repairing tissue and maintaining day-to-day function. He argues that these hormones matter beyond keeping body fat low and keeping muscles and tendons strong and self-repairing. In his account they are also key to brain function and to keeping cognitive function throughout the lifespan.

So the big theme I'd like to introduce is that metabolism isn't just about losing weight, but having a high metabolism, provided it's not too high, is great.

The qualifier "provided it's not too high" matters to his framing. Huberman links a high metabolism to more lean tissue (bone and muscle) and less adipose tissue, or fat, and says "we know that that's healthy." He does not cite specific studies for this in this segment. Growth hormone is introduced as one of the episode's two themes, but this part of the conversation deals only with the thyroid.

Huberman places the control system in the hypothalamus. He explains that the name just means it sits below the thalamus. It lies at the base of the brain in the front, as part of the forebrain, roughly a centimeter or so above the roof of the mouth and about a centimeter forward in most people. Neurons there release what he calls releasing hormones. They extend axons, which he calls "little wires," into the pituitary. The pituitary then releases hormones into the bloodstream, most often named stimulating hormones because they stimulate organs.

Thyroid-releasing hormone in the brain tells the pituitary to act. The pituitary releases thyroid-stimulating hormone. The thyroid, which he describes as a little butterfly-shaped gland right around the Adam's apple with four little bumps behind it that he calls the parathyroid gland, binds the stimulating hormone. The thyroid then releases two hormones, T4 and T3, into the blood to stimulate different tissues and their metabolism.

He promises listeners the scheme is "really easy": releasing hormone comes from the brain, stimulating hormone comes from the pituitary, and the target gland responds. He then simplifies further, telling listeners they can basically forget about T4. He hedges this right away: T4 is "not completely inactive" and "has some roles," but T3 is the one that is "more or less active."

And guess what? You can basically forget about T4.

Huberman says the main role of T3 is to promote metabolism. He stresses that this means the use of energy, including the buildup of tissues, and not only the consumption of energy. He lists target tissues including muscle, the liver, cartilage and bone. At the cellular level, he says thyroid hormone helps break fats down into fatty acids and convert them into ATP, which he calls important for cells to use energy. It also helps turn sugars into energy.

He says the body has different kinds of fat, which he plans to cover later in the episode. Thyroid hormone acts on white fat, where it liberates, or helps liberate, some of the fat from fat cells to be used for energy. He gives this as the reason higher thyroid is associated with leaner bodies and lower thyroid with less lean bodies. He frames this as an association and does not present it as a dose-response rule.

The first actionable point he names is iodine. He says the thyroid needs it to produce thyroid hormone, and that iodine is most common in sea salt, kelp and seaweed. He takes a reassuring line here. Most people can get enough iodine from the food they eat and/or the table salt they consume, and almost all table salt worldwide contains iodine.