What Does the Thyroid Do?

The thyroid is a butterfly-shaped gland located at the front of your neck, below the Adam's apple. It produces two primary hormones — triiodothyronine (T3) and thyroxine (T4) — that influence nearly every organ in your body.

Key Functions

• Metabolism — regulates the rate at which your body burns calories and uses energy; determines your basal metabolic rate.
• Heart rate and cardiac output — thyroid hormones directly affect how fast your heart beats and how strongly it contracts.
• Body temperature — helps maintain thermal homeostasis by adjusting heat production.
• Brain development — critical for fetal and infant brain development; congenital hypothyroidism causes irreversible intellectual disability if untreated.
• Growth — works with growth hormone to regulate skeletal and tissue development in children.
• Cholesterol metabolism — influences how your liver processes LDL cholesterol.

The Feedback Loop

Your thyroid doesn't act alone. The hypothalamus releases TRH, signaling the pituitary to release TSH, which then tells the thyroid to release T3 and T4. When T3/T4 levels are adequate, TSH production is suppressed — a negative feedback loop. Lab testing (TSH + free T4 + free T3) reveals where in this loop a dysfunction occurs.

When the Thyroid Malfunctions

• Too little hormone (hypothyroidism) — everything slows down: fatigue, weight gain, cold intolerance, brain fog.
• Too much hormone (hyperthyroidism) — everything speeds up: anxiety, rapid heart rate, weight loss, heat intolerance, tremor.
• Nodules and goiter — lumps or enlargement of the gland itself, which may or may not affect hormone levels.