Thyroid Tests in Mental Health: Uncovering Hidden Causes

A patient walks into a psychiatrist’s office complaining of persistent low mood, fatigue, difficulty concentrating, and a creeping sense of hopelessness. The clinical picture looks like depression – but is it? In a meaningful proportion of such cases, the real culprit is not a mood disorder at all. It’s the thyroid gland. This is why thyroid function tests (TFTs) have become an indispensable part of physical investigation in mental health assessments. Understanding what these tests measure, when to use them, and what to do with their results can be the difference between a misdiagnosis and a treatable cause.

What thyroid function tests actually measure 🔗

TFTs are a panel of blood tests used to evaluate how well the thyroid gland is working. The key markers include thyroid-stimulating hormone (TSH), produced by the pituitary gland to regulate thyroid output; thyroxine (T4), the main hormone secreted by the thyroid; and triiodothyronine (T3), the more biologically active form of thyroid hormone. TSH is typically the first-line screening test. When the thyroid is underactive, TSH rises as the pituitary tries to stimulate more hormone production. When it’s overactive, TSH drops because the feedback loop signals the pituitary to slow down. Free T4 and free T3 (unbound, biologically active fractions) give a more direct picture of hormone availability in the body. In psychiatric settings, these tests are often ordered alongside anti-thyroid antibodies – such as anti-thyroid peroxidase (anti-TPO) – to detect autoimmune thyroid disease, which adds another layer of complexity to the thyroid-mental health relationship.

Thyroid dysfunction mimicking mental illness 🔗

The thyroid-brain connection is not incidental. Thyroid hormones play a significant role in regulating mood, cognition, and neuronal activity, and disruptions at either end of the spectrum – too little or too much – can produce psychiatric symptoms that closely resemble primary mental disorders. This overlap is so pronounced that studies have shown patients are frequently misdiagnosed or delay seeking care due to the psychiatric appearance of their symptoms.

Hypothyroidism and depression 🔗

When the thyroid is underactive, the result is insufficient circulating thyroid hormone – a state that has a well-documented impact on mood and cognition. The most commonly reported psychological symptoms of hypothyroidism include forgetfulness, fatigue, mental sluggishness, inattention, and emotional instability, with depression being the predominant affective disorder. In severe hypothyroidism, the clinical picture can even resemble melancholic depression or dementia – a phenomenon historically described as “myxedema madness”, first coined by Asher in 1949, where psychosis accompanied profound thyroid deficiency. Anxiety disorders occur in approximately 60% of hyperthyroid patients, while depressive disorders have been found in 31-69% of those with thyrotoxicosis. Even subclinical hypothyroidism – where TSH is mildly elevated but hormone levels appear normal – carries real psychiatric risk. Research indicates that acquired hypothyroidism may lead to depression through several mechanisms, including disruptions in serotonergic neurotransmission and activation of neuroinflammatory pathways.

Hyperthyroidism and anxiety or mania 🔗

At the other extreme, an overactive thyroid floods the body with excess hormone, and the psychiatric consequences are equally striking. The classical neuropsychiatric presentation of hyperthyroidism includes dysphoria, anxiety, irritability, emotional lability, and impaired concentration – symptoms that closely mirror a generalised anxiety disorder or a manic episode. In older adults, hyperthyroidism can paradoxically present with apathy, depression, and cognitive decline rather than agitation – making it even harder to identify without a blood test. Mania specifically has a recognised link to hyperthyroid states, and research has shown significant differences in thyroid function between patients with bipolar disorder experiencing manic episodes compared to those in depressive phases.

The critical clinical implication is straightforward: TFTs should be ordered in all new-onset psychiatric presentations, particularly mood disorders, anxiety, cognitive changes, or psychosis, before attributing symptoms to a primary psychiatric cause. Treating a thyroid disorder can resolve or significantly reduce what appeared to be a mental illness – without any psychiatric medication at all.

Lithium and thyroid monitoring: a non-negotiable requirement 🔗

For patients already in psychiatric care – particularly those being treated for bipolar disorder – the thyroid story doesn’t end with the initial workup. Lithium, one of the most effective and widely used mood stabilisers in psychiatry, has a well-established and clinically significant impact on thyroid function. Any patient on lithium requires regular TFT monitoring throughout the course of treatment.

How lithium affects the thyroid 🔗

Lithium interferes with thyroid hormone production and release at multiple points. It inhibits the synthesis and secretion of thyroid hormones, decreases peripheral conversion of T4 to T3, and increases the risk of thyroid autoimmunity – particularly in genetically susceptible individuals. The net result is that lithium pushes the thyroid toward underactivity. A large UK-based study found that lithium use was significantly associated with an increased risk of hypothyroidism, and the authors recommended monitoring for thyroid dysfunction – particularly in young women, who appear most vulnerable. Goitre (enlargement of the thyroid gland) is also a recognised complication of long-term lithium therapy.

How often should TFTs be checked in lithium patients? 🔗

Thyroid function tests – including serum TSH, free T4, free T3, and anti-thyroid antibodies – should be performed at baseline before starting lithium, and then monitored at regular intervals throughout therapy. Current guidance recommends checks every 3-6 months during the first year of lithium therapy, followed by monitoring every 6-12 months thereafter. Lithium treatment increases the risk of clinical hypothyroidism up to fivefold, with the risk being particularly elevated in women aged 40-59 years. Higher-risk patients – including middle-aged women, those with a family history of thyroid disease, and those who test positive for anti-thyroid antibodies – may require more frequent monitoring, every 3-4 months.

What to do when lithium causes hypothyroidism 🔗

An important clinical point: baseline TFTs should be measured prior to starting lithium to ensure that undetected hypothyroidism is not already contributing to mood symptoms. When lithium-induced hypothyroidism does develop, lithium does not need to be stopped. Most clinical guidelines recommend continuing lithium and adding levothyroxine (T4 replacement therapy) to correct the hormonal deficit. There is also evidence that thyroid supplementation may help with treatment-refractory or rapidly cycling bipolar disorder, where subclinical hypothyroidism may be contributing to mood instability. The key is identifying the problem early – which is only possible through consistent monitoring.

Beyond T3/T4: advanced thyroid tests in psychiatry 🔗

Standard TFTs – TSH, free T4, and free T3 – cover the vast majority of what clinicians need in psychiatric practice. However, two more specialised tests have historically attracted interest in psychiatry research: the TRH stimulation test and the dexamethasone suppression test (DST). Both probe the neuroendocrine axes that connect brain function to hormonal regulation, and both have found a place – albeit a limited one – in the investigation of mood disorders.

The TRH stimulation test 🔗

The thyrotropin-releasing hormone (TRH) stimulation test involves administering synthetic TRH intravenously and measuring the TSH response. In healthy individuals, TRH triggers a predictable rise in TSH. In some patients with depression, this TSH response is blunted – a finding that suggests dysregulation of the hypothalamic-pituitary-thyroid (HPT) axis even in the absence of overt thyroid disease. Research has found that a blunted TRH stimulation test occurred in 25% of patients with endogenous depression and 44% with bipolar disorder in its depressed phase – pointing to a neuroendocrine fingerprint distinct from thyroid disease itself. However, the sensitivity of this test for depression is inconsistent across studies, and its specificity is limited. Its clinical utility in everyday psychiatric practice is therefore constrained, and it is largely confined to research settings rather than routine diagnosis.

The dexamethasone suppression test 🔗

The dexamethasone suppression test (DST) operates on a different axis – the hypothalamic-pituitary-adrenal (HPA) axis, which governs the cortisol stress response. In this test, a dose of dexamethasone (a synthetic steroid) is given the evening before, and cortisol levels are measured the following day. In healthy individuals, dexamethasone suppresses cortisol production. In a subgroup of severely depressed patients, this suppression fails – a phenomenon known as cortisol non-suppression. The APA Task Force on Laboratory Tests in Psychiatry noted that the DST has a sensitivity of approximately 40-50% in major depression, rising to 60-70% in very severe or psychotic affective disorders, but that its clinical utility remains limited due to false positives from medical conditions, weight loss, alcohol use, and certain medications. Like the TRH test, the DST has generated substantial research interest but has not found a stable place in routine clinical practice. The variability in results and susceptibility to confounding factors make both tests supplementary tools rather than standard diagnostic instruments.

It is worth noting that the DST was initially reported with a diagnostic sensitivity of 67% and specificity of 95% for melancholic depression, generating considerable enthusiasm – but subsequent studies produced widely varying results, leading the American Psychiatric Association to ultimately reject it as a routine clinical tool. Both tests remain scientifically valuable for probing neuroendocrine mechanisms, but clinicians should not rely on them for diagnosis in the way standard TFTs are used.

The practical takeaway 🔗

Thyroid function tests sit at a genuinely important intersection of physical medicine and psychiatry. A simple blood draw measuring TSH and free thyroid hormones can reveal a biological cause behind what looks, on the surface, like depression, anxiety, or mania. For patients starting lithium, these tests shift from diagnostic to preventive – catching drug-induced thyroid dysfunction before it silently worsens the mood disorder being treated. More advanced tests like the TRH stimulation test and dexamethasone suppression test add nuance to our understanding of how the brain’s hormonal systems go wrong in mental illness, even if they are not yet ready for routine clinical use. Taken together, thyroid function testing is not a box-ticking exercise. It is a meaningful part of ensuring that no treatable physical cause is missed behind a psychiatric label.

What do you think? If a patient had been treated for depression for years with antidepressants but had never had their thyroid checked, how different might their clinical journey have looked? And given how significantly lithium affects thyroid function, do you think current monitoring guidelines go far enough in protecting patients on long-term mood stabiliser therapy?

References

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