Thyroid test results

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Thyroxine, free (FT4)
• When taking no thyroid hormones or when taking thyroid hormone
extract (also referred to as natural dessicated thyroid or NDT), such as
Armour®, optimal FT4 is approximately 60% of range (1.4 ng/dL when
range is .82-1.77 ng/dL)
• When taking NDT, low FT4 may indicate low cortisol or not enough
thyroid medication
• FT4 results at top of or over range, hyperthyroidism should be ruled
• When taking T3-only hormones, expect FT4 level to be suppressed; if
not suppressed, hyperthyroidism should be ruled out
• FT4 can measure high when estrogen levels are higher from
pregnancy, birth control pills or estrogen replacement therapy
Triiodothyronine, free (FT3)
• When taking no thyroid hormones or when taking NDT, optimal FT3
level is in top quarter of the range.
• When taking optimal dose T3-only meds, FT3 result is usually over
range by 10 to 25%
• High FT3 on low-dose of T3-only meds can indicate the hormone is
not being utilized; look at iron and/or cortisol
Reverse T3 (RT3)
• RT3 is always compared to FT3 done from the same blood draw
• Ratio calculated between FT3 and RT3 must be at least 20
• Ratio of less than 20 indicates T4 is preferentially converting to RT3
• When taking T3-only thyroid hormone, expect RT3 to be very low and
no longer a useful measure
Calculating FT3/RT3 ratio
When the units used to measure the two substances are in the same
kind of measure, calculating a ratio is straightforward. When one
substance is measured in moles (e.g., nmol/L) and the other is not
(e.g., ng/dL), conversion is first necessary. Multiply ng/dL by .0154 to
find nmol/L. Or, just use the ratio worksheet here
Note: If it is impossible to test RT3, if iron is low and FT3 is considerably lower
in its range than FT4 is in its, one can safely surmise an RT3 issue.
Thyroid antibodies (TPOAb and TgAb)
• Over-range results indicate thyroid is under current autoimmune
• Any antibodies indicate Hashimoto's Thyroiditis; low levels indicate no
current attack
• More common for TPO to be elevated
• TgAb can be elevated when TPO is not
Thyroid stimulating hormone (TSH)
See discussion here