Levothyroxine absorption interactions are measurable: in one 12-week trial, taking iron alongside the dose moved mean TSH from 1.6 to 5.4 mU/L.

TSH moving on levothyroxine is rarely treatment failure. It peaks near 2 a.m. and bottoms out in the early afternoon, so the hour of your draw matters.

Levothyroxine TSH retest timing is biology, not scheduling: steady state arrives at about six weeks. Every dose change restarts that clock.

Preparing for a TSH test? Timing is the variable most people miss: in one study, median TSH fell from 5.83 to 3.79 mU/L between morning and afternoon.

TSH test timing moves results more than expected: 14.1% of 7-8am samples read
above 4.0 mIU/L, against 7.9% at 2-3pm in the same clinic.

Biotin before a thyroid test: a multivitamin's 30 mcg has not been reported to interfere. A 10,000 mcg gummy is 10 mg — the case-report amount.

TSH normal range in the elderly is not what your lab prints: in women the upper limit rises from 4.0 mIU/L at 50 to 6.0 by 90, while free T4 barely moves.

TSH targets before pregnancy split women into two groups, and only one needs to
be under 2.5 mU/L. The 2026 guideline explains which is which.

TSH levels in pregnancy are read on a different scale. When mildly raised results were rechecked at about three weeks, only 41% were still raised.

The normal TSH range by age is not what your lab used. In NHANES data, 70% of older adults with a TSH above 4.5 sat within their age-specific range.