TSH over 10 mIU/L

Recommendation 15 of the AACE/ATA guideline: "Patients whose serum TSH levels exceed 10 mIU/L are at increased risk for heart failure and cardiovascular mortality, and should be considered for treatment with L-thyroxine." The grade is B, downgraded because the meta-analysis behind it "does not include prospective interventional studies." The narrative states the consensus more plainly: "there is general agreement that patients with primary hypothyroidism with TSH levels above 10 mIU/L should be treated." The 2014 ATA treatment guideline reports the same shape of evidence, with cardiovascular risk "primarily observed in individuals with TSH levels >10 mIU/L, with potential effects of TSH values >7 mIU/L."

What the number does and does not say. A TSH above 10 does not by itself mean overt hypothyroidism; the guideline defines overt disease as "An elevated TSH, usually above 10 mIU/L, in combination with a subnormal free T4." Free T4 is the test that settles which it is (Recommendation 7 says free T4, not total T4, outside pregnancy, and Recommendation 10 says T3 should not be used to diagnose hypothyroidism). Two things can push TSH into this band without thyroid failure: recovery from a non-thyroidal illness, when the guideline says TSH can rise to levels "generally below 20 mIU/L," and assay interference from heterophilic antibodies, which cause "falsely elevated serum TSH values." Adrenal insufficiency can also raise TSH and reverses with glucocorticoid replacement.

How treatment starts. The guideline's standard is levothyroxine alone (Recommendation 22.1, grade A); it finds no evidence for T4 plus T3 combinations and says desiccated thyroid "should not be used." The dose depends on the person. Full replacement is about 1.6 µg/kg a day and is what young, healthy adults with overt disease may start on (Recommendation 22.7.1), while people older than 50 to 60 without coronary disease start at 50 µg (Recommendation 22.7.2), and the 2014 ATA guideline says anyone with known coronary artery disease "should always be started on a low LT4 dose (12.5–25 µg/d)." A person whose free T4 is still normal is dosed like the subclinical band, 25 to 75 µg. The tablet is taken "with water consistently 30–60 minutes before breakfast or at bedtime 4 hours after the last meal," away from calcium, iron and other interfering products.

The follow-up rhythm. Recommendation 13: TSH "at 4–8 weeks after initiating treatment or after a change in dose," then at 6 months once the dose is settled, then every 12 months. The 2014 guideline uses 4 to 6 weeks for the same step and adjusts in increments of 12.5 to 25 µg. The target is the lab's normal range, with no preferred spot inside it (Recommendation 27), and the 2014 guideline notes the target may reasonably be raised to 4 to 6 mIU/L past age 70 to 80, because older adults are "more susceptible to the adverse effects of thyroid hormone excess, especially atrial fibrillation" and fractures. Starting an interacting drug means remeasuring TSH within 4 to 8 weeks (Recommendation 26), and the guideline lists pregnancy, cardiac disease, a goiter or nodule, and difficulty reaching a stable state among the reasons to involve an endocrinologist.