Depression can have a biochemical signature. So can thyroid disease, iron deficiency, and cortisol dysregulation. Those signatures can overlap in ways that can be clinically meaningful and frequently missed - not because clinicians aren't careful, but because the standard depression workup may not include the labs that can catch them.
This is not an argument against antidepressants. It is an argument for biological completeness before treatment. A woman presenting with fatigue, low mood, anhedonia, and disrupted sleep may have major depressive disorder, hypothyroidism, ferritin below 20 ng/mL, or elevated evening cortisol. The treatment for each may be different. Running the labs first costs less than a mismatched prescription and a year of wondering why it isn't working.
Mimic 1: Thyroid - especially Free T3
Hypothyroidism may be the closest biochemical analogue to depression. Slowed metabolism can produce exactly the constellation that fills DSM criteria: low energy, slowed thinking, reduced motivation, flat affect, impaired concentration, weight gain, disturbed sleep. When mild - normal TSH, low-normal Free T3 - it can be invisible on a standard workup.
The symptom profile that can distinguish thyroid from primary depression: cold intolerance, constipation, hair thinning at the outer third of the eyebrows, dry skin, slow pulse, and hoarseness. These may not be present, but their clustering alongside mood symptoms should prompt a full thyroid cascade, not just TSH.
Normal TSH can't rule out thyroid-driven mood symptoms
Free T3 - the active hormone - can be low with a normal TSH in the setting of chronic stress, dieting, or Hashimoto's in its early stages. A full panel including Free T3 may be necessary. Levothyroxine augmentation may offer benefit in depression even in patients with normal TSH in multiple small trials.
Mimic 2: Cortisol dysregulation
The HPA axis and the mood regulatory system are not separate circuits - they are deeply intertwined. Elevated evening cortisol can suppress melatonin, fragment sleep, and increase amygdala reactivity (emotional hyperarousal). Blunted morning cortisol, a possible hallmark of HPA exhaustion, can produce anhedonia and motivational flatness that can be phenomenologically indistinguishable from melancholic depression.
Critically, standard depression screening - PHQ-9, HAM-D - may not distinguish between primary mood disorder and HPA dysregulation. Both can produce the same scores. A four-point diurnal cortisol test can characterise the underlying pattern. Treatment implications can diverge: HPA dysregulation can respond to circadian anchoring, adaptogenic support, and stress-load reduction; primary depression can require psychiatric intervention.
Mimic 3: Iron - specifically ferritin
Dopamine and serotonin synthesis both require iron as a cofactor - specifically in the hydroxylase enzymes that produce these neurotransmitters. Iron deficiency can impair monoamine metabolism at the root. The result can be a neurochemical environment that resembles what antidepressants are designed to correct - but one that may not respond to SSRIs because the substrate problem may be upstream of the serotonin transporter.
A 2003 study found that depression scores in women with ferritin below 20 ng/mL may be statistically indistinguishable from those with clinical depression on the Beck Depression Inventory. Correcting ferritin to functional levels (above 50 ng/mL) can improve mood scores significantly in some trials - not as a cure for depression, but as a resolution of the biochemical floor that can prevent recovery.
The full workup before escalating treatment
- TSH + Free T3 - screen for hypothyroidism and low-T3 syndrome.
- Ferritin - a target may be >50 ng/mL; below 20 may be significant in a mood context.
- 4-point salivary cortisol - characterise the diurnal pattern before attributing to primary depression. EllaDx doesn't offer salivary cortisol testing. You can order morning cortisol as a blood test, drawn at Quest.
- Vitamin D (25-OH) - deficiency may be strongly associated with SAD and can contribute to low mood year-round.
- Vitamin B12 - deficiency can produce neuropsychiatric symptoms including depression and cognitive impairment.
This is not either/or
Biological mimics and primary psychiatric depression can coexist. Hashimoto's and depression may co-occur more frequently than chance. Low ferritin and PMDD can overlap. Correcting the biochemical substrate may not cure depression - but missing it can mean every antidepressant trial runs with one hand tied. The baseline labs are not an alternative to psychiatric care; they are the floor it rests on.
