Stress, Metabolism and Growth: Adrenal, Thyroid and Growth Hormones

The moment before the puck drops

It's the last minute of a tied playoff game, or you're waiting outside the room for a job interview. Your heart pounds, your mouth goes dry, your hands get sweaty, and you feel suddenly very awake. That's your stress response: an ancient system that prepares the body to act.

In this lesson you will see that the body has two stress systems, one fast and one slow. Then you will study two other hormones that set the pace of the whole body: thyroid hormone, which controls metabolic rate, and growth hormone. All three follow the same pattern you learned last lesson: hypothalamus → pituitary → gland, with negative feedback.

Short-term stress: fight or flight

When the brain senses a threat, the hypothalamus activates the sympathetic nervous system. Sympathetic nerves stimulate the adrenal medulla (the inner part of each adrenal gland) to release epinephrine (adrenaline) and norepinephrine into the blood within seconds. Their effects:

• heart rate and stroke volume increase, so blood pressure rises;
• airways (bronchioles) widen, so more air reaches the alveoli;
• the liver breaks down glycogen, so blood glucose rises;
• blood flow shifts toward skeletal muscles and away from the digestive system and skin;
• pupils dilate.

These effects fade within minutes once the threat is gone, because epinephrine is broken down quickly.

Long-term stress: cortisol and the HPA axis

If stress lasts longer (an illness, an injury, weeks of exams), a slower hormone chain takes over. This is the hypothalamus–pituitary–adrenal (HPA) axis:

1. The hypothalamus releases CRH (corticotropin-releasing hormone).
2. CRH causes the anterior pituitary to release ACTH.
3. ACTH stimulates the adrenal cortex (outer part) to release cortisol.

Cortisol is a steroid hormone. It:

• raises blood glucose by stimulating gluconeogenesis in the liver (using amino acids from protein breakdown);
• helps the body use fats for energy;
• reduces inflammation and immune activity.

Cortisol also feeds back to inhibit the hypothalamus and pituitary, so less CRH and ACTH are released: negative feedback. Cortisol normally follows a daily rhythm too, highest in the morning to help you wake up.

In the short term, cortisol helps us cope. But if stress stays high for a very long time, persistently raised cortisol can affect sleep, blood pressure, blood glucose and resistance to infection. Regular physical activity, enough sleep, time with friends and talking to someone you trust all help the stress system return to baseline. That's not just advice; it is homeostasis.

Medical treatments that act on the HPA axis

Corticosteroid medicines, such as prednisone, are similar to cortisol. Doctors prescribe them to calm inflammation in conditions like severe asthma flare-ups, arthritis and some allergic reactions. They work very well, but they also act on the HPA axis: the body "sees" high cortisol-like levels, so by negative feedback the pituitary releases less ACTH. After weeks of treatment, the adrenal cortex can become less active. That is why a doctor will often taper (gradually lower) the dose rather than stopping it suddenly, giving the adrenal glands time to start making cortisol again.

Two uncommon disorders show what happens when cortisol is out of balance: Cushing's syndrome (too much cortisol, from a tumour or long-term high-dose medication) and Addison's disease (the adrenal cortex makes too little cortisol and aldosterone; it is treated with hormone replacement).

The thyroid: setting the body's pace

The thyroid gland wraps around the front of the windpipe below the larynx. It makes thyroxine (T₄) and triiodothyronine (T₃), which contain iodine: four atoms in T₄ and three in T₃. They are controlled by the hypothalamus–pituitary–thyroid axis: TRH → TSH → T₄ and T₃, with T₄ and T₃ feeding back to inhibit TRH and TSH.

Thyroid hormones enter almost every cell and increase the metabolic rate: more mitochondrial activity, more oxygen use and more heat production. They are also essential for normal growth and brain development in babies, which is why newborns in Ontario are routinely screened for thyroid problems.

• Hypothyroidism (too little thyroid hormone): tiredness, feeling cold, weight gain, dry skin, slow heart rate. A common cause is an autoimmune condition (Hashimoto's thyroiditis). Treatment is a daily tablet of levothyroxine, a synthetic T₄, with blood tests to adjust the dose.
• Hyperthyroidism (too much): feeling hot, sweating, weight loss, rapid heartbeat, nervousness. A common cause is Graves' disease, in which antibodies bind to the TSH receptor and stimulate the thyroid like TSH does, ignoring feedback. Treatments include anti-thyroid medicines, radioactive iodine or surgery.
• Iodine deficiency: without iodine, the thyroid can't make enough T₄ and T₃. This leads to an enlarged thyroid, a goitre. Canada requires table salt to be iodized, which is why iodine-deficiency goitre is rare here.

Growth hormone

Growth hormone (GH) from the anterior pituitary stimulates growth of bones (at the growth plates near the ends of long bones), muscles and other tissues, largely through another hormone the liver makes in response. GH also raises blood glucose and helps the body use fat. It is released in pulses, especially during deep sleep, which is one reason sleep matters for growing teens.

• Too little GH in childhood: slow growth and short stature. Since the 1980s, children can be treated with GH made by recombinant DNA technology.
• Too much GH in childhood, before the growth plates close: gigantism (very tall stature).
• Too much GH in adulthood, after the growth plates have closed: acromegaly, in which bones of the hands, feet and face thicken.

Both excesses are usually caused by a benign tumour of the pituitary.