A child with silky fine hair may have a head full of kinky curls when puberty hits. Another baby may start out with fine ringlets only to have them turn stick straight when she gets older.

As our body ages, we see changes in our hair texture. The hair of a newborn is very soft and very fine. The diameter of the hair thickens as we enter childhood or early adolescence. As we move into adulthood and then older, the hair again changes, becoming finer again in our 40s and 50s. All of these changes are genetically programmed.

“Hair changes every five to seven years,” said Christo of the Christo Fifth Avenue Salon and creator of the Curlisto line of hair products.

But even though we might know what affects hair texture, it is still unclear exactly how and why this happens. People with curly hair have a flatter follicle. Straight hairs tend to have a rounder follicle. But how a flat follicle turns round, and vice versa, is a mystery.

“What adds to the curl or takes away from the curl is a fascinating issue scientifically,” says Tom Dawson, a principle scientist at P&G Beauty. “You’d think with something as fundamental as human hair, and the amount of time and money we put into our hair, that we’d know more than we do. But it’s a tough nut to crack.

“What is known is that hormones have a major effect on hair texture. Hormones influence the hair in several different ways since they are regulators of the body’s metabolism,” Christo says. “Hormones tell our body how to perform, therefore increasing or decreasing specific hormones can directly affect the way our body functions.”

Hormones and your hair

He cites three major hormones that affect the hair:

Thyroxine and triiodothyronine

Thyroxine and triiodothyronine are produced by the thyroid gland and influence the way the body uses the food we eat. An under-active thyroid can cause brittle hair and hair loss.


Androgen is produced in the adrenal glands and is responsible for hair growth. In women, an increase in the production of androgen causes thinning of the hair and excessive facial hair.


Insulin is produced in the pancreas and is responsible for the adaptation of glucose in the blood. Under-production of insulin can cause hyperglycemia and over-production can cause hypoglycemia, which can both cause hair loss and change the way the hair looks and feels.

Hair after chemo

Cancer patients often find that after chemotherapy, their once-straight hair grows in curly. Chemo affects rapidly growing cells more than slowly growing ones, and hair follicles in the scalp grow rapidly. They are jolted by the chemo, and when they go back to work, they may have a new job description, says Dr. Jennifer Griggs. Over time, the hair follicle tends to return to its normal shape.

With little scientific evidence available about how hormones and genetics cause these texture changes, Jonathan Torch, creator of Curly Hair Solutions and founder of Toronto’s Curly Hair Institute, has come up with his own theory.

He believes changes in the muscles at the base of every follicle are the key to the changes in hair texture that take place over time. These muscular changes, he says, often come during puberty, chemotherapy or menopause, when hormones and medications may affect the muscle tone.

Sometimes these changes can be extreme, says Torch, who has witnessed many a client go from curly to straight and straight to curly.”I can’t prove anything medically,” he says. “But I have a philosophy that genetically, the muscles are changing. And this changes the shape of the follicle.”

Much like the changes we experience with our skin as children through puberty and into adulthood, hormonal changes in hair texture are a normal and natural process throughout the course of a woman’s life.

Have you seen a drastic change in your hair? Did it grow curlier or less curly with age? 

This article was originally published in December 2007 and has been updated for grammar and clarity.

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