Depression Due to Hypothyroidism
Her TSH is high enough to warrant thyroid treatment on its own — but subclinical hypothyroidism's actual link to depression is contested enough that treating the thyroid alone is a real gamble on a four-month, functionally disabling episode.
S.D., a 45-year-old bank branch manager, has spent the past four months feeling progressively more fatigued, unmotivated, and low, to the point of turning down a regional promotion she had been actively pursuing for over a year — a decision her husband describes as completely out of character. She sought care specifically for these mood symptoms, not for any physical complaint, and her PHQ-9 today is 19, with prominent fatigue, poor concentration, and anhedonia.
Routine bloodwork drawn at that same visit came back with a TSH of 8.2 mIU/L and a free T4 within the normal range — subclinical hypothyroidism, found incidentally rather than through any thyroid-specific symptom she had reported. She has no prior thyroid history, no other chronic illness, and no family history of thyroid disease that she's aware of. This is her first depressive episode of any severity in her life.
The actual clinical uncertainty here is more specific than 'hypothyroidism causes depression.' Overt hypothyroidism, with a clearly elevated TSH and low free T4, has a well-established association with depressive symptoms that often improve with thyroid hormone replacement alone. Subclinical hypothyroidism — elevated TSH with a still-normal free T4, which is what she has — carries a much weaker and more inconsistent link to mood symptoms across the literature, with some studies showing a modest association and others showing none. Subclinical hypothyroidism is also normally confirmed on a repeat TSH some weeks later, because a single elevated value is often transient — and starting levothyroxine now forecloses that confirmation permanently, since every subsequent TSH reflects the drug. Starting levothyroxine alone and waiting the four to six weeks it typically takes to reach a stable euthyroid state, then reassessing her mood, also risks leaving a functionally disabling depression untreated for over a month on a mechanism that may not even be driving her symptoms — while starting an antidepressant concurrently risks attributing improvement to the wrong intervention if her mood does track her thyroid correction instead.
At the primary care follow-up visit
I'd treat the thyroid, though I want to be precise about why: at 8.2 she is below the TSH of 10 where replacement is firmly indicated on its own, so this is a discretionary trial in a symptomatic patient under 65 rather than a clear-cut indication. TPO antibodies would help — positive antibodies push a TSH in her range toward treating. And if the thyroid does turn out to be contributing to her mood, that's a genuinely reversible cause worth addressing.
I agree the thyroid should be treated either way, but I don't think we should wait to see if it explains her mood before starting anything for the depression itself. A PHQ-9 of 19 with a declined promotion and four months of progressive functional decline is a real, disabling episode, and the subclinical-hypothyroidism-and-depression literature is genuinely mixed enough that betting her recovery timeline on it resolving with thyroid correction alone feels like the wrong risk to take.
If we start both now and her mood improves, we won't know cleanly which intervention did it — but that's a smaller cost than leaving a functionally disabling depression untreated for the four to six weeks levothyroxine needs to reach steady state.
Starting both concurrently is defensible specifically because the two timelines don't actually conflict — levothyroxine takes four to six weeks to reach stable euthyroid levels, and sertraline takes a comparable window to show meaningful effect. The one genuine interaction to watch is that SSRIs have been reported to raise levothyroxine requirements, so a TSH that drifts up later shouldn't be read as non-adherence without considering the sertraline. I'd document clearly that attribution will stay genuinely uncertain if she improves, and build a plan to test that uncertainty later rather than pretend the answer is already known.
Levothyroxine and sertraline were started on the same day, with TPO antibodies sent before the first dose, TSH rechecked at six weeks, and PHQ-9 reassessed at the same visit. The team documented explicitly that treating now means the original TSH will never be confirmed on an untreated repeat — a real cost accepted deliberately for the sake of her mood timeline.
The team will not be able to cleanly separate which intervention drove the improvement — a real, acknowledged limitation of treating concurrently, documented as such rather than presented as resolved.
That result would argue against subclinical hypothyroidism as a meaningful driver in her case specifically, and would shift the reasoning toward sertraline and standard depression management going forward.
The team agreed the honest cost of treating concurrently was diagnostic clarity, not patient safety — a tradeoff made deliberately given how disabling four months of untreated depression already was, not overlooked in the decision.