Thyroid Scan

A thyroid scan is a nuclear medicine imaging test that uses a small amount of radioactive material to produce pictures of the thyroid gland — the butterfly-shaped gland at the front of your neck that controls your body's metabolism and energy use.

You swallow or receive an injection of a radioactive tracer (a harmless, short-lived substance that emits a faint signal). The tracer travels through your bloodstream and is absorbed by thyroid tissue. A special camera called a gamma camera then detects the signal and maps how different parts of the gland are working. Areas that absorb too much or too little tracer can point to specific problems.

Doctors order a thyroid scan when blood tests or an ultrasound suggest there is something abnormal about how the thyroid gland looks or functions. It helps them see not just the size and shape of the gland but also how active each part is.

• Hyperthyroidism (an overactive thyroid) — to find out whether the whole gland or only a part of it is producing too much hormone
• Thyroid nodules (lumps inside the thyroid) — to determine whether a nodule is 'hot' (very active) or 'cold' (less active, which may need further investigation)
• Goitre (an enlarged thyroid gland) — to assess the size and activity of the gland
• Suspected ectopic thyroid tissue — thyroid tissue that has grown in an unusual location
• Follow-up after thyroid cancer treatment — to check whether any thyroid tissue or cancer cells remain
• Unexplained neck swelling that may involve the thyroid

A thyroid scan is generally not used in certain situations. Your doctor will consider your individual circumstances before recommending it.

• Pregnancy — the radioactive tracer can harm the developing baby, so the scan is avoided unless there is a serious reason with no safer alternative
• Breastfeeding mothers — breastfeeding is usually paused for a period after the scan because the tracer can pass into breast milk
• People who have recently had scans using iodine-containing contrast dye (used in CT scans) — the dye can interfere with tracer absorption and give unreliable results
• People currently taking thyroid hormone medication or certain other medicines that affect iodine uptake — these are usually paused first, under medical guidance

A thyroid scan is a low-risk diagnostic procedure. The amount of radioactive material used is very small, and it leaves the body naturally within a day or two. Most people experience no side effects at all. Recognised risks include:

• Very low radiation exposure — the dose is comparable to or less than many routine X-rays; long-term effects at this level are considered extremely unlikely
• Mild allergic reaction to the tracer — rare, and usually limited to brief skin flushing or a mild rash
• Temporary metallic taste or mild nausea after swallowing the radioactive iodine capsule — usually short-lived
• Discomfort at the injection site if the tracer is given intravenously (through a vein)
• Risk to pregnancy — as noted above, the scan is avoided in pregnant women because the thyroid of a developing baby can absorb the tracer

Preparation for a thyroid scan is focused mainly on making sure the thyroid is ready to absorb the tracer properly. Your medical team will give you specific instructions, but the following points reflect common practice.

Diet and lifestyle changes: Many hospitals ask patients to follow a low-iodine diet (limiting seafood, iodised salt, and dairy) for one to two weeks before the scan, because dietary iodine can compete with the tracer and reduce image quality. Alcohol does not usually need to be avoided specifically, but following a clean diet is generally encouraged. Smoking is not known to directly affect the scan, but your team may have specific advice.

Medications: Several medicines can interfere with the scan. These commonly include thyroid hormone tablets, anti-thyroid drugs (medicines that slow down the thyroid), certain heart medicines, amiodarone (a heart medicine high in iodine), and supplements containing iodine or kelp. Your doctor will advise which, if any, need to be paused and for how long — never stop a medication without medical guidance.

Pre-scan tests: Blood tests to measure thyroid hormone levels (T3, T4) and TSH (thyroid-stimulating hormone) are usually done beforehand. These help the nuclear medicine team interpret the scan images accurately.

The scan itself typically follows these steps:

• You arrive at the nuclear medicine department and confirm your identity and medical history with the team
• The radioactive tracer is given — either as a capsule you swallow or as an injection into a vein in your arm, depending on the type of tracer used
• You wait for the tracer to be absorbed by the thyroid; this waiting period is usually several hours if radioactive iodine is used, or around twenty to thirty minutes if a different tracer (technetium) is used
• You lie still on a flat table while the gamma camera moves slowly around your neck and chest area to capture images — this part is painless and usually takes around twenty to forty minutes
• The technologist checks the image quality before you leave; if additional images are needed, they will be taken at the same visit or a follow-up visit
• You are free to go home once imaging is complete

Because the radioactive tracer leaves the body quickly, recovery after a thyroid scan is straightforward for most people. There is no wound to care for and no anaesthesia (medication to make you unconscious) involved. However, there are a few sensible precautions to follow in the hours and days after the scan.

• Drink plenty of water after the scan to help flush the tracer from your body through urine more quickly
• If radioactive iodine was used, your hospital may advise brief precautions around close or prolonged contact with pregnant women, infants, and young children — usually for one to a few days; your team will give specific guidance
• Breastfeeding mothers should follow the team's instructions about when it is safe to resume breastfeeding
• Flush the toilet twice after urinating for the first day or two, as a simple hygiene measure
• You can usually eat and drink normally right after the scan, unless your doctor advises otherwise
• Most people can drive and return to normal daily activities immediately after the scan
• Your scan images will be reviewed by a nuclear medicine specialist (a doctor trained in reading these images) and the results sent to your referring doctor, usually within a few days
• A follow-up appointment will be arranged to discuss the results and decide on the next steps in your care