Low TSH and low free T4: when the problem is the signal, not the thyroid

You open a lab report and see two flags next to each other: TSH below the range, and free T4 below the range too. If you have read anything about the thyroid, this looks wrong. A low TSH usually goes with too much thyroid hormone, not too little. So what does it mean when both are low?

This article explains why this combination is read differently from the usual thyroid results, what the possible explanations are, and what doctors check next.

This material is for information only and does not replace a consultation with a doctor. A thyroid result is interpreted by a doctor together with your symptoms, medicines and other tests.

Why is "low and low" unusual?

TSH is a signal hormone. It is made by the pituitary gland at the base of the brain and tells the thyroid how much T4 to produce. In the most common thyroid problems the fault is in the thyroid itself, and the pituitary reacts: when T4 falls, TSH rises; when T4 is in excess, TSH falls.

When TSH is low and free T4 is low, that reaction is missing. The thyroid is making too little hormone, yet the signal that should push it harder is not rising. That shifts attention away from the thyroid and towards the signal itself, or towards something that is changing the numbers.

What can produce this pattern?

A problem with the signal: central hypothyroidism

In central hypothyroidism the thyroid gland is normal, but it is not stimulated enough because the pituitary, the hypothalamus above it, or both are not working properly. A review in Nature Reviews Endocrinology (2017) describes it as rare. It can occur on its own or together with shortages of other pituitary hormones, and that review describes those other shortages as mostly acquired rather than inborn. Its symptoms are usually milder than in the common form of hypothyroidism.

The same review states the key point for reading a report: the diagnosis rests on low free T4 with a low or normal TSH. That means TSH can look normal here. A lab strategy that measures only TSH and adds free T4 only when TSH is abnormal can miss central hypothyroidism altogether.

Being seriously unwell: non-thyroidal illness

During serious illness, thyroid tests change even when the thyroid is healthy. A 2015 review in The Lancet Diabetes & Endocrinology describes the typical picture in intensive-care patients: low T3, low T4, and a TSH that is normal or slightly decreased. This is called non-thyroidal illness syndrome. Its extent is associated with prognosis, but, as the authors write, there is no proof that the low hormone levels cause the worse outcome.

Treating the numbers has been tested. In a small randomized trial published in 1986, 23 intensive-care patients with low T4 received either intravenous thyroxine for two weeks or no thyroxine. Mortality was the same in both groups: 75% without treatment and 73% with it. The authors concluded that thyroxine was not beneficial in this group. What this does not say: it was a small trial in critically ill patients, and it says nothing about people with central hypothyroidism.

Medicines that turn TSH down

Some medicines suppress TSH at the level of the pituitary or hypothalamus. A 2009 review lists glucocorticoids, dopamine agonists, somatostatin analogues and rexinoids (a class used in some cancer treatment). According to the review, most of these do not cause clinically evident central hypothyroidism, while rexinoids cause clinically significant central hypothyroidism in most patients who take them. This is why your doctor will ask what you take before interpreting the result.

A measurement problem

Laboratory tests can be distorted. Many thyroid tests use a biotin–streptavidin reaction, and biotin is sold as a supplement. In a small study, six healthy adults took 10 mg of biotin a day for a week. Interference appeared in 9 of 23 assays that used this reaction and in none of the 14 assays that did not. Depending on the assay design, some results came out falsely high and others falsely low. This was a preliminary study in six people, but it is enough to make one practical point: tell the doctor and the lab if you take biotin or a "hair, skin and nails" supplement.

What does a doctor usually check next?

There is no single test that settles it, so the next steps usually follow the possible causes:

• Repeat the test and look at the context: recent illness, hospital stay, medicines and supplements.

• Other pituitary hormones. The 2018 European Thyroid Association guideline says that a fault in the cells that make TSH is frequently part of a shortage of several pituitary hormones, not of TSH alone. How those hormones are replaced in adults is the subject of a separate Endocrine Society guideline (2016).

• Imaging of the pituitary area, if the hormone picture points there. This is decided by an endocrinologist.

If central hypothyroidism is confirmed, free T4 is the number used to judge the dose, because TSH itself is the faulty signal. The 2018 European Thyroid Association guideline, written by experts in the absence of randomized trials, follows free T4 rather than using TSH as the dose target. TSH is not ignored: the same guideline says to measure it when too little or too much replacement is suspected, and reads a TSH above 1.0 mU/L with a free T4 near the bottom of the range as a sign that the dose may be too low. The same guideline says thyroid hormone is started only once cortisol secretion has been shown to be adequate, or with steroid treatment if a cortisol shortage has not been ruled out, because starting thyroid hormone first can precipitate an adrenal crisis. In newborns and infants who are already symptomatic, it says thyroid hormone should not be delayed. It also says overt central hypothyroidism, with free T4 below the range, should be recognised and treated at any age. For milder forms, where free T4 is still inside the lower end of the normal range, it recommends not treating people older than 75. That milder group is not the pattern in this article.

What to do with your result

• Do not start, stop or change thyroid tablets on the basis of this combination alone. The same two numbers have very different explanations.

• Bring the full report to your doctor: TSH, free T4, and if available free T3, together with a list of medicines and supplements, including biotin.

• If you were recently seriously ill, mention it: the result may need to be repeated once you have recovered.

• If the doctor suspects the pituitary, expect tests of other hormones before any treatment decision.

You can follow your TSH and free T4 over time in your Lonevi health map, which makes it easier to see whether a single result is a trend or a one-off.

Sources

• Persani L et al. 2018 European Thyroid Association (ETA) Guidelines on the Diagnosis and Management of Central Hypothyroidism. Eur Thyroid J. 2018. PMID 30374425

• Beck-Peccoz P et al. Central hypothyroidism – a neglected thyroid disorder. Nat Rev Endocrinol. 2017. PMID 28549061

• Fliers E et al. Thyroid function in critically ill patients. Lancet Diabetes Endocrinol. 2015. PMID 26071885

• Brent GA, Hershman JM. Thyroxine therapy in patients with severe nonthyroidal illnesses and low serum thyroxine concentration. J Clin Endocrinol Metab. 1986. PMID 3011834

• Haugen BR. Drugs that suppress TSH or cause central hypothyroidism. Best Pract Res Clin Endocrinol Metab. 2009. PMID 19942154

• Li D et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017. PMID 28973622

• Fleseriu M et al. Hormonal replacement in hypopituitarism in adults: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2016. PMID 27736313