The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. T4 and T3 control your metabolism, while calcitonin helps regulate calcium levels in your blood. Of these, T4 makes up roughly 80% of the thyroid’s hormonal output, with T3 accounting for the remaining 20%.
T4 and T3: The Metabolic Hormones
T4 and T3 are often lumped together as “thyroid hormone” because they work as a team. Your thyroid produces both using iodine from food, but their roles are different. T4 is largely inactive on its own. It circulates through your bloodstream as a precursor, and organs like the liver and kidneys convert it into T3, the active form that actually enters your cells and influences how they use energy.
This conversion step is important because it lets your body fine-tune how much active hormone is available at any given time. Rather than flooding every tissue with the active form, your thyroid ships out mostly T4 and lets individual organs decide how much T3 they need locally. In infants, thyroid hormone is critical for brain development, making adequate levels especially important during pregnancy and early life.
What Thyroid Hormones Do in Your Body
Thyroid hormone touches nearly every system in your body. Its central job is setting the pace of your metabolism, the rate at which your cells burn fuel and produce energy. When thyroid levels are high, your resting energy expenditure increases, cholesterol drops, fat breakdown accelerates, and you tend to lose weight. When levels are low, the opposite happens: energy expenditure slows, cholesterol rises, and weight gain becomes easier.
Beyond metabolism, thyroid hormone influences your body in several specific ways:
- Heart and circulation: T3 affects heart rate and how forcefully your heart contracts. Too much thyroid hormone speeds the heart; too little slows it.
- Blood sugar: Thyroid hormones stimulate both glucose production in the liver and insulin-driven glucose uptake by cells, keeping blood sugar regulation in balance.
- Fat processing: In the liver, local T3 levels govern fat metabolism. Disruptions can increase susceptibility to fatty liver disease and obesity.
- Muscle function: Higher T3 levels in skeletal muscle shift the type of muscle fibers present, influence muscle regeneration, and increase energy expenditure.
- Body temperature: Thyroid hormone activates heat production in brown fat tissue, helping your body maintain its core temperature.
- Brain and appetite: In the hypothalamus, thyroid hormone availability helps regulate feeding behavior and overall energy balance.
Thyroid hormone also drives mitochondrial activity, the energy-producing process inside every cell. This is one reason why people with an underactive thyroid often feel profoundly fatigued: their cells are literally generating less energy at the molecular level.
Calcitonin: The Calcium Regulator
Calcitonin comes from a different set of cells within the thyroid called C-cells, or parafollicular cells. These are distinct from the cells that produce T4 and T3, and calcitonin serves a completely different purpose: lowering calcium levels in your blood.
It does this in two ways. First, calcitonin temporarily blocks the cells that break down bone (releasing calcium into the bloodstream in the process). By slowing bone breakdown, less calcium enters circulation. Second, calcitonin reduces the amount of calcium your kidneys reabsorb, so more calcium leaves through urine instead of cycling back into your blood. The overall effect is a modest downward push on blood calcium levels, complementing the work of parathyroid hormone, which raises calcium when it drops too low.
How Your Body Controls Thyroid Secretion
Your thyroid doesn’t decide on its own how much hormone to release. It takes orders from a chain of command that starts in your brain. The hypothalamus, a small region at the base of the brain, produces a signaling molecule called TRH. TRH travels to the pituitary gland, which sits just below it, and tells the pituitary to release TSH (thyroid-stimulating hormone). TSH then travels through the bloodstream to the thyroid and triggers it to produce and release T4 and T3.
The system keeps itself in check through a feedback loop. When T4 and T3 levels in the blood rise high enough, they signal back to both the hypothalamus and pituitary to ease off. The hypothalamus produces less TRH, the pituitary produces less TSH, and the thyroid slows its output. When hormone levels drop, the brakes come off and production ramps back up. This feedback loop keeps T4 and T3 within a narrow range, which is why even small shifts outside that range can produce noticeable symptoms.
Normal TSH levels generally fall between 0.5 and 5.0 mIU/L, and free T4 typically ranges from 0.7 to 1.9 ng/dL. Doctors use these values to gauge whether the feedback loop is functioning properly. A high TSH with low T4, for example, suggests the pituitary is working overtime to stimulate a thyroid that isn’t keeping up.
When Secretion Goes Wrong
Because thyroid hormones influence so many systems, overproduction and underproduction create recognizably different patterns. An overactive thyroid (hyperthyroidism) produces a hypermetabolic state: rapid heart rate, weight loss despite normal appetite, anxiety, heat intolerance, and sometimes tremors. An underactive thyroid (hypothyroidism) does the reverse: fatigue, weight gain, cold sensitivity, sluggish thinking, and elevated cholesterol.
Calcitonin abnormalities are far less common in daily life, but calcitonin levels can serve as a diagnostic marker. Unusually high calcitonin sometimes signals medullary thyroid cancer, which originates in the C-cells that produce it. Outside of that context, calcitonin plays a relatively modest role compared to the outsized influence of T4 and T3 on day-to-day health.