What does a low TSH mean?

Overt hyperthyroidism means a suppressed TSH with high T3 and/or free T4 [1]. Subclinical hyperthyroidism means a persistently low TSH with free hormones inside their reference intervals [2]. The ETA grades it as grade 1 when TSH is 0.1 to 0.39 mIU/L and grade 2 when it is below 0.1 [2]. Large pooled cohorts set the lower limit of normal at 0.45 mIU/L [8].

NICE recommends that when TSH is below the reference range, the lab measures both free T4 and free T3 in the same sample [3]. That cascade is what catches T3 toxicosis, where free T4 is normal but T3 is high.

Free T4 is reported in ng/dL in the US and pmol/L elsewhere; 1 ng/dL is about 12.9 pmol/L [9]. Compare your value with the range on your own report, because free T4 assays differ.

Thyroid causes

Graves disease is the most common cause of hyperthyroidism, with a global prevalence of about 2% in women and 0.5% in men [1]. Other causes include toxic nodules and the thyrotoxic phase of thyroiditis [1]. Endogenous subclinical hyperthyroidism usually comes from Graves disease, autonomously functioning nodules or multinodular goitre [2].

Thyroid hormone treatment

People taking levothyroxine can have a low TSH if the dose is more than they need. Overtreatment is linked with cardiovascular risk, which is why TSH is checked 6 to 8 weeks after a dose change and then yearly [10]. Only the prescribing clinician should change a dose.

Causes outside the thyroid

• Illness: critically ill people often have normal or slightly low TSH with low T3 and T4 [5].
• Medicines: glucocorticoids, dopamine agonists, somatostatin analogues and rexinoids can suppress TSH [6].
• Biotin: high doses can make TSH read falsely low on some assays [7].
• Pituitary disease: NICE advises measuring free T4 with TSH when pituitary disease is suspected, because TSH alone can mislead [3].
• Pregnancy: hormonal changes in early pregnancy lower TSH, which is why pregnancy uses trimester-specific ranges [11][9].

The JAMA review by Lee and Pearce lists anxiety, insomnia, palpitations, unintentional weight loss, diarrhea and heat intolerance as common symptoms of thyrotoxicosis [1]. Graves disease can also cause an enlarged thyroid, a stare or bulging eyes, and toxic nodules can press on the neck and cause trouble swallowing or voice changes [1].

Symptoms overlap with anxiety, menopause and many other conditions, so they cannot confirm or rule out thyroid disease on their own. NICE notes that one symptom alone may not indicate thyroid disease [3].

• Atrial fibrillation: in Framingham adults aged 60 and over, TSH of 0.1 mU/L or lower gave a 28% ten-year incidence of atrial fibrillation versus 11% with normal TSH [4]. Pooled cohorts found a hazard ratio of 1.68 for atrial fibrillation with endogenous subclinical hyperthyroidism, highest below 0.10 mIU/L [8].
• Mortality: the same pooled data showed a hazard ratio of 1.24 for total mortality [8].
• Fractures: TSH below 0.10 mIU/L carried hazard ratios of 1.61 for hip fracture and 1.98 for any fracture [12].

The ETA recommends treatment for people over 65 with grade 2 subclinical hyperthyroidism and says it could be considered for grade 1 in that age group [2]. For younger people without symptoms and with grade 1, it advises monitoring without treatment [2].

1. Free T4 and free T3 from the same blood sample, which NICE recommends whenever TSH is below range [3].
2. A repeat test. For untreated subclinical hyperthyroidism, NICE suggests measuring TSH every 6 months, with free T4 and free T3 if it is outside the range [3]. NICE's specialist-advice threshold uses two readings below 0.1 mIU/L at least 3 months apart [3].
3. Finding the cause. The cause can usually be established from the clinical picture, thyroid function tests and TSH receptor antibodies; a thyroid scan is recommended if nodules are present or the cause is unclear [1].
4. Checking for interference. Mention biotin and steroid use, since both can lower TSH without thyroid disease [7][6].

Treatment choices, such as antithyroid drugs, radioactive iodine or surgery, belong to a clinician and depend on the cause [1][13].

Thyroid storm is a rare but life-threatening form of extreme thyrotoxicosis with multi-organ dysfunction; it needs high-dependency or intensive care [14]. Untreated hyperthyroidism can also cause arrhythmias and heart failure [1].

A mildly low TSH found on a routine test, with no symptoms, is not an emergency. It is a reason for a repeat test and a conversation with your clinician.