Thyroid Function Tests (TFTs)—often searched as TFT or “thyroid functioning tests”—are a group of blood tests that check how well your thyroid gland is working. The thyroid is a small gland in the front of the neck that helps control your metabolism (how your body uses energy), temperature, heart rate, digestion, and many aspects of mood and energy.

TFTs typically measure:

• TSH (thyroid-stimulating hormone): Made by the pituitary gland in the brain. TSH tells your thyroid how much hormone to produce. It is usually the most sensitive single marker for thyroid function.
• Free T4 (thyroxine): One of the main thyroid hormones. “Free” T4 is the active portion available to tissues (not bound to proteins).
• Free T3 (triiodothyronine): The more active thyroid hormone at the tissue level. It may be especially helpful when hyperthyroidism is suspected or when T3 levels change earlier than T4.

In plain language, results are often interpreted like this (your clinician will interpret based on your symptoms, medications, pregnancy status, and reference ranges):

• High TSH + low Free T4: Suggests hypothyroidism (underactive thyroid).
• Low TSH + high Free T4 and/or high Free T3: Suggests hyperthyroidism (overactive thyroid).
• High TSH + normal Free T4: Often called subclinical hypothyroidism (milder underactivity).
• Low TSH + normal Free T4: Often called subclinical hyperthyroidism (milder overactivity).
• Normal TSH with abnormal T4/T3: May occur with certain medications, pituitary problems, serious illness (“non-thyroidal illness”), or changes in thyroid-binding proteins—your clinician may order additional tests to clarify.

Depending on the reason for testing, your provider may add related labs such as thyroid antibodies (TPOAb, TgAb, TRAb/TSI) to look for autoimmune thyroid disease, or thyroglobulin for specific thyroid cancer follow-up.

Doctors commonly order TFTs to diagnose or monitor thyroid conditions, explain symptoms that may be caused by thyroid hormone imbalance, or to adjust thyroid medication dosing. Testing may be recommended if you have symptoms of an underactive or overactive thyroid, a thyroid lump/goiter, or risk factors for thyroid disease.

Common reasons to check TFTs include:

Symptoms that may suggest hypothyroidism (low thyroid function): fatigue, weight gain, feeling cold, constipation, dry skin, hair thinning or hair loss, puffy face, slowed heart rate, depression/low mood, memory or concentration problems (“brain fog”), heavy or irregular periods, fertility concerns.

Symptoms that may suggest hyperthyroidism (high thyroid function): anxiety/irritability, unexplained weight loss, heat intolerance, sweating, tremor, palpitations or fast heartbeat, frequent bowel movements/diarrhea, trouble sleeping, muscle weakness, lighter or irregular periods.

Other situations where TFTs are often ordered:

• Goiter or thyroid nodules: To see whether a thyroid growth is associated with over- or underproduction of hormones.
• Pregnancy and postpartum: Thyroid balance is important for maternal health and fetal development; testing may be done if symptoms, past thyroid disease, type 1 diabetes, or other risk factors are present. Postpartum thyroiditis can also occur after delivery.
• Monitoring known thyroid disease: To adjust treatment for hypothyroidism (e.g., levothyroxine) or hyperthyroidism (antithyroid drugs, after radioactive iodine, or after thyroid surgery).
• Irregular heart rhythm or cardiovascular concerns: Especially new atrial fibrillation, persistent tachycardia, or unexplained changes in cholesterol (hypothyroidism can raise LDL cholesterol).
• Unexplained changes in weight, energy, or temperature tolerance: When lifestyle factors alone don’t explain the change.
• Medication effects: Some medicines can affect thyroid labs or thyroid function (for example, amiodarone, lithium, interferon, high-dose biotin supplements can interfere with certain lab assays). Always tell your clinician about supplements and medications.

If you are taking high-dose biotin (often found in hair/skin/nail supplements), ask your clinician whether you should stop it briefly before testing, because it can cause misleading TFT results in some lab methods.

• Hypothyroidism (underactive thyroid)
• Hyperthyroidism (overactive thyroid)
• Hashimoto’s thyroiditis (autoimmune hypothyroidism)
• Graves’ disease (autoimmune hyperthyroidism)
• Subclinical hypothyroidism
• Subclinical hyperthyroidism
• Thyroiditis (e.g., subacute thyroiditis, postpartum thyroiditis)
• Goiter (enlarged thyroid)
• Thyroid nodules (including “toxic” nodules)
• Pituitary or hypothalamic disorders causing central hypothyroidism (less common)

• Thyroid screening and early detection of thyroid hormone imbalance (TFT/thyroid functioning check)
• Explaining persistent fatigue, brain fog, mood changes, or temperature intolerance
• Supporting healthy weight management by ruling out thyroid-related causes
• Fertility planning and menstrual cycle health (including preconception evaluation when indicated)
• Pregnancy and postpartum wellness (monitoring thyroid changes that can affect mother and baby)
• Cardiovascular health monitoring (palpitations, atrial fibrillation risk, and cholesterol changes linked to thyroid function)
• Medication monitoring and dose adjustment for levothyroxine or antithyroid therapy
• General wellness check when symptoms suggest hormonal imbalance or metabolic changes