Understanding Your Thyroid — Function, Causes of Dysfunction, and When to Get Tested

What Is the Thyroid, and What Does It Do?

Your thyroid is a small, butterfly-shaped gland at the front of your neck, under the skin. It’s part of your endocrine system, and its main job is to control your metabolic rate — how your body converts the food you eat into energy. The thyroid gland regulates many metabolic processes, including growth and energy expenditure, which is why even a small imbalance can affect how you feel day to day.

Thyroid dysfunction is more common than most people realize — around 1 in 20 people will experience some form of it in their lifetime. Most thyroid conditions have an autoimmune basis and a genetic link, so a family history of thyroid conditions raises your own risk.

Iodine’s Role in Thyroid Health

Iodine is a mineral your thyroid needs to produce hormones, but your body doesn’t make its own — it has to come from food. Natural sources include seaweed, shrimp, and other seafood, along with some fortified dairy products. In many countries, iodized table salt (introduced widely since the 1920s) has been the main driver behind falling rates of iodine deficiency. Too little iodine can lead to hypothyroidism, and in some cases, a visible enlargement of the thyroid gland called a goitre.

How the Pituitary Gland Controls Your Thyroid

The thyroid doesn’t work alone — it’s controlled by the pituitary gland, often called the ‘master gland’ of the endocrine system. The pituitary produces thyroid-stimulating hormone (TSH), which tells the thyroid how much T3 and T4 hormone to release. If T4 levels rise too high, the pituitary reduces TSH output to slow the thyroid down; if T4 is too low, it increases TSH to speed things up. This feedback loop, overseen by the hypothalamus, is what keeps hormone levels stable — and it’s exactly what a TSH blood test measures.

Common Causes of Thyroid Dysfunction

• Iodine deficiency — the most common cause of hypothyroidism where iodized salt isn’t widely available; often causes a visible goitre
• Hashimoto’s disease — a chronic autoimmune condition and the leading cause of hypothyroidism where iodine intake is adequate
• Congenital hypothyroidism — present from birth, affecting roughly 1 in 2,000 to 4,000 babies
• Graves’ disease — a chronic autoimmune condition that causes hyperthyroidism (an overactive thyroid), often with symptoms like weight loss, rapid heartbeat, and anxiety
• Thyroid nodules or inflammation (thyroiditis) — can temporarily or permanently affect hormone output

When Should You Get a Thyroid Test?

Consider testing if you notice unexplained fatigue, weight changes, hair thinning, feeling unusually cold or hot, irregular periods, or mood changes that don’t have an obvious cause. Pregnant women, people with a family history of thyroid conditions, and those with other autoimmune diseases are also good candidates for routine screening.