TSH above 4.12 and 10 or lower mIU/L

The AACE/ATA guideline defines this band against your laboratory's range, not against 4.12: "Subclinical hypothyroidism is characterized by a serum TSH above the upper reference limit in combination with a normal free thyroxine (T4)." It attaches three conditions before the label applies at all: "This designation is only applicable when thyroid function has been stable for weeks or more, the hypothalamic–pituitary–thyroid axis is normal, and there is no recent or ongoing severe illness." A single TSH drawn during an illness or in its recovery phase, when the guideline says TSH can rise "generally below 20 mIU/L," does not qualify.

The decision, in the guideline's words. Recommendation 16: "Treatment based on individual factors for patients with TSH levels between the upper limit of a given laboratory's reference range and 10 mIU/L should be considered particularly if patients have symptoms suggestive of hypothyroidism, positive TPOAb or evidence of atherosclerotic cardiovascular disease, heart failure, or associated risk factors for these diseases." The grade is B, and the guideline explains the downgrade: "there are no prospective, interventional studies." Its narrative is blunter about the middle of this band: which patients at 4.5 to 10 "will benefit is less certain," and for 2.5 to 4.5 "there are virtually no clinical outcome data."

The antibody test. Recommendation 1 says thyroid peroxidase antibodies "should be considered when evaluating patients with subclinical hypothyroidism," and the guideline gives the reason as a rate: with positive antibodies "hypothyroidism occurs at a rate of 4.3% per year versus 2.6% per year when anti-thyroid antibodies are negative." It adds that the result "may or may not influence the decision to treat."

Age cuts the other way. The cardiovascular evidence the guideline reviews splits at 65: studies with a mean age under 65 showed a coronary risk ratio of 1.51 for subclinical hypothyroidism, versus 1.05 at 65 and over, and a second meta-analysis found elevated odds "for those under 65 years, but not for those over 65 years." Combined with the age drift in the reference range itself, that is why the guideline warns that "not all patients who have mild TSH elevations are hypothyroid and therefore would not require thyroid hormone therapy." The guideline does not number a recommendation to repeat the TSH before treating in this band; the "stable for weeks or more" condition in its definition is the closest it comes.

If treatment is chosen. Recommendation 22.8: "initial L-thyroxine dosing is generally lower than what is required in the treatment of overt hypothyroidism. A daily dose of 25–75 µg should be considered, depending on the degree of TSH elevation." The TSH is rechecked 4 to 8 weeks after starting or changing a dose (Recommendation 13; the 2014 ATA treatment guideline says 4 to 6), then at 6 months and yearly, with the target being the lab's normal range. Pregnancy is the exception to everything above: the guideline treats women planning or in early pregnancy at lower TSH values, and lists pregnancy and planned conception among the reasons to see an endocrinologist. Its 2012 pregnancy cutoffs have since been replaced by the ATA's 2017 and 2026 pregnancy guidelines.