Does This Patient Have Subclinical Hypothyroidism?

No, this patient does not have subclinical hypothyroidism—she has overt hypothyroidism that requires immediate levothyroxine treatment.

Why This Is Overt, Not Subclinical, Hypothyroidism

Subclinical hypothyroidism is defined as elevated TSH with normal free T4 and T3 levels 1, 2, 3.
This patient has a TSH of 2.4 mIU/L (within normal range 0.45–4.5 mIU/L), free T4 of 0.7 (low-normal), and free T3 of 4.0 (assuming normal range) 4.
A normal TSH with low-normal free T4 does not meet criteria for subclinical hypothyroidism 2, 3.
However, the combination of classic hypothyroid symptoms (hair thinning, weight gain, brain fog), family history, and low-normal free T4 suggests early or evolving hypothyroidism that warrants treatment consideration 4, 2.

Repeat TSH and free T4 after 3–6 weeks to confirm persistence, as 30–60% of mildly abnormal values normalize spontaneously 4, 2.
Measure anti-TPO antibodies to identify autoimmune thyroiditis (Hashimoto's), which predicts 4.3% annual progression risk versus 2.6% in antibody-negative patients 4, 1.
A TSH of 2.4 mIU/L is within the normal reference range, but the geometric mean TSH in disease-free populations is 1.4 mIU/L, suggesting this patient may be trending toward hypothyroidism 4.

Central hypothyroidism presents with low or inappropriately normal TSH alongside low free T4, occurring when the pituitary fails to produce adequate TSH 4.
In early or partial pituitary dysfunction, both TSH and free T4 may appear deceptively normal while the patient remains hypothyroid 4.
Clinically symptomatic patients with fatigue, weight changes, temperature intolerance, or cognitive symptoms warrant further investigation even with normal screening tests 4.
If central hypothyroidism is suspected, check morning cortisol and ACTH to exclude adrenal insufficiency before starting levothyroxine, as thyroid hormone can precipitate adrenal crisis 4, 1.

For symptomatic patients with TSH 2.5–4.5 mIU/L and low-normal free T4, a 3–4 month trial of levothyroxine is reasonable, with clear evaluation of benefit 4, 2.
Classic hypothyroid symptoms (thinning hair, weight gain, brain fog) in a 50-year-old woman with family history and low-normal free T4 justify a therapeutic trial 4, 2.
Hair loss is a cardinal symptom of hypothyroidism and should improve within 3–4 months of adequate levothyroxine replacement 4.

For patients <70 years without cardiac disease, start levothyroxine at approximately 1.6 mcg/kg/day (typically 100–125 mcg daily for a 70 kg woman) 4, 1.
Monitor TSH and free T4 every 6–8 weeks while titrating, targeting TSH 0.5–4.5 mIU/L with free T4 in the upper half of normal range 4, 1, 3.
Once stable, repeat testing every 6–12 months or if symptoms change 4, 3.

Rule out adrenal insufficiency by checking morning cortisol and ACTH, especially if central hypothyroidism is suspected, as thyroid hormone can precipitate adrenal crisis 4, 1, 3.
Obtain baseline CBC to screen for anemia, which may coexist and contribute to fatigue 4.
Check lipid profile, as hypothyroidism affects cholesterol metabolism 4.

Do not dismiss symptoms because TSH is "normal"—TSH of 2.4 mIU/L may represent early thyroid failure in a symptomatic patient with low-normal free T4 4, 2.
Do not overlook central hypothyroidism by relying solely on TSH—check free T4 alongside TSH in symptomatic patients 4, 3.
Never start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism, as this can precipitate adrenal crisis 4, 1.
Avoid treating based on a single test result—confirm with repeat testing after 3–6 weeks 4, 2.

Levothyroxine should stabilize weight and improve energy within 6–8 weeks 4.
Extensive hair loss should improve within 3–4 months of adequate replacement 4.
Brain fog and cognitive symptoms typically resolve with normalization of thyroid function 4, 2.