Fatigue, cold intolerance, and a low TSH in early pregnancy
A 29-year-old woman at 9 weeks gestation presents with 6 weeks of fatigue, constipation, and cold intolerance. She has no prior thyroid history and takes only a prenatal vitamin. Exam: pulse 58, BP 104/66, dry skin, delayed relaxation of the ankle reflex, small diffuse goiter without nodules. Labs: TSH 0.18 mIU/L (ref 0.4–4.0), free T4 0.7 ng/dL (ref 0.8–1.8), total T4 low-normal, thyroglobulin antibodies negative, TSH-receptor antibodies negative. She asks whether her thyroid is 'overactive' because the TSH is low and whether she should start medication.
1. Interpret the thyroid panel. Why is TSH low when the patient is clearly hypothyroid clinically, and how does pregnancy alter interpretation?
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The pattern is central (secondary) hypothyroidism, not hyperthyroidism: low free T4 with an inappropriately low rather than elevated TSH points to pituitary/hypothalamic failure. In primary hypothyroidism TSH would be high.
Pregnancy confounders to weigh:
- hCG cross-reactivity at the TSH receptor mildly lowers TSH in the first trimester, but it does not lower free T4; a genuinely low free T4 is not explained by hCG.
- Increased thyroid-binding globulin raises total T4, so total T4 is unreliable; free T4 (or free T4 index) is the interpretable value, and trimester-specific ranges are lower than non-pregnant ranges.
- The low pulse, delayed reflex relaxation, and goiter support hypothyroidism rather than thyrotoxicosis.
The key judgment: a low TSH does not exclude hypothyroidism. The paired low free T4 makes this central hypothyroidism until proven otherwise.
2. What else must be evaluated before or alongside starting levothyroxine, and why does the order matter in pregnancy?
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Central hypothyroidism raises two safety issues:
- Adrenal insufficiency must be excluded first. If the cause is panhypopituitarism (e.g., Sheehan, lymphocytic hypophysitis - which can present in pregnancy/postpartum, or a pituitary lesion), starting levothyroxine without glucocorticoid coverage can precipitate an adrenal crisis by accelerating cortisol clearance. Check morning cortisol (or cosyntropin stimulation) and consider pituitary MRI and other anterior pituitary axes (prolactin, IGF-1, LH/FSH).
- Do not titrate to TSH. In central hypothyroidism TSH is unreliable; dosing is guided by free T4 targeted to the upper part of the trimester-specific range, plus clinical response.
Also review the prenatal vitamin for iron/calcium, which impair levothyroxine absorption (separate dosing by ≥4 hours), and coordinate with obstetrics because untreated maternal hypothyroidism is associated with impaired neurodevelopment.
3. Six weeks after starting levothyroxine 75 mcg daily, free T4 is mid-normal, TSH is 0.9, and she feels better but still constipated. What is the appropriate next step and what is the reasoning?
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Do not increase the dose reflexively on the basis of residual constipation alone while free T4 is therapeutic. In central hypothyroidism the therapeutic target is free T4 in the upper reference range for gestation, with clinical assessment; a mid-normal free T4 with residual symptoms permits a modest increase (e.g., 12.5–25 mcg) with recheck in 4–6 weeks, but the decision is driven by free T4, not TSH.
Reasoning points:
- TSH 0.9 here is uninformative; chasing it risks overtreatment.
- Constipation in pregnancy has many causes (iron in prenatal vitamins, progesterone-induced dysmotility, low fiber/fluid); evaluate those rather than assuming under-replacement.
- Overtreatment risks in pregnancy include preterm labor and fetal effects, so avoid pushing free T4 above the trimester range.
- Confirm adherence and separation from iron/calcium before escalating dose.