Nursing care
Thyroid Function Tests: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
Thyroid function tests measure TSH, free T4 and free T3 to judge how the thyroid gland is working. TSH moves in the opposite direction to thyroid hormone: a high TSH means the gland is underactive, and a low TSH means it is overactive. Getting that reversal backwards is the single most common error nurses and students make when reading these results.
What the test measures
A thyroid panel usually reports three numbers: thyroid-stimulating hormone (TSH), free T4, and sometimes free T3. TSH is released by the anterior pituitary and drives the thyroid gland to produce T4 and T3. T4 is the storage form; it converts to the more active T3 in peripheral tissue.
The pituitary and thyroid work on a feedback loop. When circulating thyroid hormone drops, the pituitary senses it and raises TSH to push the gland harder. When thyroid hormone is abundant, the pituitary lowers TSH to ease off. TSH is therefore not a measure of thyroid hormone itself; it is a measure of how hard the pituitary is having to work to get a normal result out of the gland. That is why it moves in the opposite direction to the hormone it is chasing.
Normal ranges and what moves them
TSH typically runs about 0.4 to 4.0 mIU/L in adults, though the exact cutoff varies by lab and by patient population (pregnancy lowers the acceptable range, older adults tolerate a slightly higher one). Free T4 sits roughly between 0.8 and 1.8 ng/dL. Reference ranges differ enough between assays that a result should always be read against the lab's own reported range, not a number memorised from a textbook.
Results can shift for reasons that have nothing to do with thyroid disease. Acute illness, starvation, high-dose corticosteroids, and some psychiatric medications can all suppress TSH temporarily (sick euthyroid syndrome). Biotin supplements, common in hair and nail products, interfere with the immunoassay itself and can produce a falsely low TSH with a falsely high free T4. Ask about biotin use before treating an unexpected result as real.
What a high result means
A high TSH means the gland is underactive and the pituitary is compensating by shouting louder. This is hypothyroidism, most commonly from Hashimoto's thyroiditis, prior thyroid surgery, or radioactive iodine treatment. Free T4 will be low or low-normal alongside the raised TSH. Expect the clinical picture to match: fatigue, cold intolerance, weight gain, constipation, bradycardia, and coarse, dry skin.
A high free T4 with a high TSH is unusual and points to a pituitary source, such as a TSH-secreting adenoma, rather than primary thyroid disease. This pattern is rare but worth flagging rather than assuming a lab error. In straightforward primary hypothyroidism, the higher the TSH climbs, the more severe the hormone deficit tends to be, which is useful when judging urgency at the bedside.
What a low result means
A low TSH means the pituitary has backed off because there is already plenty of thyroid hormone in circulation, or because something outside the pituitary-thyroid axis is suppressing it. Primary hyperthyroidism, most often Graves' disease or a toxic nodule, gives a low TSH with a high free T4 and free T3. Expect tachycardia, tremor, heat intolerance, weight loss despite a good appetite, and anxiety.
A low TSH with a normal or low free T4 suggests subclinical hyperthyroidism, early pituitary failure, or the sick euthyroid pattern seen in critically ill patients. Amiodarone deserves specific attention here: it can cause either hyperthyroidism or hypothyroidism, and thyroid function should be checked before starting it and periodically thereafter.
Nursing actions by result
For a high TSH consistent with hypothyroidism, anticipate levothyroxine and teach the patient to take it on an empty stomach, at least 30 to 60 minutes before food, and to keep dosing consistent because absorption is easily disrupted. Monitor for signs of myxedema in severe, untreated cases: hypothermia, bradycardia, and altered mental status warrant urgent escalation, not routine follow-up.
For a low TSH consistent with hyperthyroidism, monitor heart rate and rhythm closely, since atrial fibrillation is a real risk in untreated Graves' disease, especially in older adults. If antithyroid drugs such as methimazole are started, teach the patient to report sore throat or fever immediately, since agranulocytosis is a rare but serious adverse effect. In either direction, correlate the lab value with symptoms before acting; a single abnormal TSH drawn during acute illness may not reflect true thyroid status and often warrants repeating once the patient has recovered.
Patient preparation and teaching
No fasting is required for a standard thyroid panel, and the test can be drawn at any time of day, though TSH has a mild diurnal variation and is often drawn in the morning for consistency when trending results over time. Ask specifically about biotin supplementation and hold it for at least 48 hours before the draw if the ordering provider agrees, since it is one of the few things a nurse can control that meaningfully distorts the result.
Teach patients being monitored for thyroid disease that results should be compared against their own baseline, not a single population range, and that dose changes in levothyroxine or antithyroid medication take four to six weeks to show their full effect on TSH. Reassure them that a repeat draw sooner than that will not yet reflect the new dose, which prevents unnecessary alarm and unnecessary re-testing.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our endocrine practice questions are the closest set to what this page covers.
Common questions
Why is TSH high when the thyroid is underactive?
TSH is made by the pituitary gland, not the thyroid, and its job is to push the thyroid to produce hormone. When the thyroid is failing and hormone levels drop, the pituitary raises TSH to try to compensate. So a high TSH signals an underactive gland, not an overactive one, even though the number itself is going up.
Does a normal TSH rule out thyroid disease?
In most cases yes, since TSH is the most sensitive single marker of thyroid function. The exception is central (pituitary or hypothalamic) thyroid disease, where TSH can look falsely reassuring while free T4 is genuinely low. If clinical suspicion is high despite a normal TSH, free T4 should still be checked.
How soon after starting levothyroxine should TSH be rechecked?
Wait at least four to six weeks. TSH responds slowly to dose changes because the feedback loop takes time to re-equilibrate, so testing earlier gives a result that does not yet reflect the new dose and can lead to an unnecessary dose adjustment.
Can biotin affect thyroid function test results?
Yes. Biotin, found in many over-the-counter hair, skin and nail supplements, interferes with the immunoassay used for TSH and free T4, typically producing a falsely low TSH and falsely high free T4 that can mimic hyperthyroidism. Ask about biotin use before acting on an unexpected result and consider holding it 48 hours before repeat testing.
What is sick euthyroid syndrome?
It is a pattern of abnormal thyroid function tests, usually a low T3 with a normal or mildly low TSH, seen in patients who are acutely and severely ill but whose thyroid gland is not actually diseased. The results tend to normalise once the underlying illness resolves, so treatment is aimed at the illness, not the thyroid numbers.