Thyroid 101
Its function, autoimmunity and tips to reduce your disease risk
The thyroid is a small, butterfly-shaped gland that sits at the base of the neck, just below the larynx (voice box). Despite its size, it is one of the most influential glands in the body — producing hormones that regulate nearly every cell and organ system, from your metabolism and energy levels to your mood, heart rate, and reproductive health.
To produce its hormones (T3 and T4), the thyroid relies on adequate iodine — making diet and nutrient status central to thyroid health.
The thyroid operates as part of a feedback loop involving the brain and pituitary gland, known as the hypothalamic-pituitary-thyroid (HPT) axis.
Step 1 — When the body needs more thyroid hormone, the hypothalamus releases TRH, signalling the pituitary gland to act.
Step 2 — The pituitary gland releases TSH into the bloodstream, which travels to the thyroid gland and signals it to produce hormones.
Step 3 — The thyroid produces hormones T4 and T3, which are released into the bloodstream and regulate metabolism, energy, mood, and many other bodily functions.
Step 4 — When T3 and T4 levels are sufficient, they signal back to the hypothalamus and pituitary to slow down TRH and TSH production. When levels drop too low, the cycle begins again.
Thyroid hormones T4 and T3 are released from the thyroid into the bloodstream in a ratio of 10% T3 (active form) and 90% T4 (inactive form).
Most thyroid hormone travels bound to a transport protein called thyroxine-binding globulin (TBG), with only a small free fraction — fT3 and fT4 — remaining unbound. It is this free fraction that is biologically active and able to enter cells, which is why free hormone levels are the more clinically meaningful measure on a blood test.
When thyroid hormone reaches a target tissue, it unbinds from TBG and crosses into the cell. If T4 has entered the cell, it must first be converted into active T3 before it can be used.
T3 is then taken into the nucleus — this is where the magic happens.
Inside the nucleus, T3 binds to thyroid hormone receptors on DNA, switching genes on or off — directly influencing how the cell produces energy, grows, and functions. This is how thyroid hormone regulates metabolism, mood, energy, and so much more at a cellular level.
This is why T4-only treatment (such as thyroxine/levothyroxine) doesn't work for everyone — if the body cannot efficiently convert T4 into active T3, cellular function remains impaired despite 'normal' looking blood results.
Thyroid hormones T3 and T4 act as messengers throughout the body, influencing nearly every cell and organ system.
Metabolism — regulate how efficiently your body converts food into energy, influencing weight, appetite, and body temperature
Energy & fatigue — drive cellular energy production, directly affecting how energised or exhausted you feel day to day
Brain & mood — affect neurotransmitter activity, affecting concentration, memory, and emotional regulation. Low thyroid hormone is strongly associated with depression and brain fog
Cardiovascular system — regulate heart rate and the force of each heartbeat, as well as cholesterol metabolism
Digestive system — impact gut motility, meaning thyroid dysfunction is commonly associated with constipation (hypothyroid) or diarrhoea (hyperthyroid)
Reproductive health — essential for menstrual regularity, fertility, and healthy pregnancy outcomes
Skin, hair & nails — support cell turnover and growth, with deficiency commonly presenting as dry skin, hair thinning, and brittle nails
Now that you understand how your thyroid works — what happens when the immune system turns against it?
Part 2 → Autoimmune thyroid conditions
