If you’ve been reading about your thyroid and gotten lost in the acronyms and medical terms, this is your reference page. Bookmark it.
TSH (Thyroid-Stimulating Hormone)
The signal your brain (pituitary gland) sends to your thyroid, telling it to make hormone. TSH is the standard thyroid screening test. High TSH = your brain is shouting (thyroid underperforming). Low TSH = brain has reduced signaling (thyroid making enough, or overperforming). Normal TSH doesn’t confirm adequate T3 at the cell level or absence of antibodies.
T4 (Thyroxine)
The main hormone your thyroid gland produces. Also the form provided by the most common thyroid medication, levothyroxine. T4 is the storage form — it’s inactive until your body converts it into T3.
T3 (Triiodothyronine)
The active form of thyroid hormone that your cells actually use. Made by converting T4. Every cell in your body has T3 receptors. When T3 is low at the tissue level, everything slows down — energy, cognition, temperature regulation, digestion, mood.
Free T4 / Free T3
The unbound, available fraction of T4 and T3. Thyroid hormone travels in the bloodstream attached to carrier proteins (like TBG). Only the “free” portion is available to enter cells and work. Free T3 is the most clinically relevant indicator of thyroid hormone adequacy at the tissue level.
Reverse T3 (rT3)
An inactive mirror-image of T3. Your body can convert T4 into rT3 (instead of active T3) under stress, illness, or inflammation. rT3 occupies T3 receptors without activating them — like a key that fits but won’t turn the lock.
TPO (Thyroid Peroxidase)
An enzyme your thyroid uses to make hormone. In Hashimoto’s, the immune system produces antibodies against TPO (TPOAb or anti-TPO). Elevated TPO antibodies confirm autoimmune thyroid disease.
TPOAb / Anti-TPO (TPO Antibodies)
Antibodies against thyroid peroxidase. The primary marker of Hashimoto’s. Can be elevated for years before TSH changes. Clinically-dosed selenium has repeated evidence for reducing TPO antibodies (Gärtner et al., 2002).
TgAb (Thyroglobulin Antibodies)
Antibodies against thyroglobulin, the protein your thyroid uses to store hormone. Elevated alongside or instead of TPOAb in some Hashimoto’s patients.
TBG (Thyroid-Binding Globulin)
The carrier protein that transports thyroid hormone in the blood. Estrogen raises TBG — which is why women on oral contraceptives or HRT often need higher thyroid doses, and why perimenopausal estrogen swings affect thyroid symptoms.
Hashimoto’s Thyroiditis
An autoimmune condition in which the immune system produces antibodies against the thyroid gland, leading to progressive inflammation and damage. The most common cause of hypothyroidism in iodine-sufficient countries. Affects women 7–10x more than men.
Hypothyroidism
Underactive thyroid — insufficient thyroid hormone production. Can be caused by Hashimoto’s, iodine deficiency, thyroid surgery, radioactive iodine treatment, or other causes. Symptoms include fatigue, brain fog, weight gain, cold intolerance, constipation, hair loss, and depression.
Subclinical Hypothyroidism
TSH is elevated (thyroid is struggling) but free T4 is still in the normal range. Often symptomatic. The population in which myo-inositol + selenium has been studied and shown to reduce TSH and improve well-being (Nordio & Basciani, 2017).
Overt Hypothyroidism
TSH is high and free T4 is low. This is when standard treatment (levothyroxine) is typically prescribed. The hormone shortage is now unmistakable on paper.
Levothyroxine (LT4)
The standard thyroid hormone replacement medication. Provides synthetic T4. Replaces the hormone a damaged thyroid can’t make. Does not address the autoimmune attack or T4→T3 conversion gap.
Deiodinase
The enzymes that convert T4 into active T3 (or into reverse T3). Selenium-dependent — selenium is required for deiodinase function. Deficiency in selenium impairs conversion.
Myo-Inositol
A naturally occurring sugar alcohol that acts as a second messenger in TSH signaling — helping thyroid cells respond appropriately to the pituitary’s signal. Also involved in insulin signaling. Studied in combination with selenium in subclinical hypothyroidism, with evidence for reducing TSH and improving well-being.
L-Selenomethionine
The most bioavailable organic form of selenium. The form used in the landmark antibody-reduction trials. The form in Thyrolume.
Piperine (Black Pepper Extract)
A compound from black pepper that inhibits the enzymes that break down many nutrients in the gut, increasing the time they spend in the intestinal lining and improving absorption. Used in supplements to support absorption of the other ingredients.
Goitrogens
Compounds that can inhibit iodine uptake into the thyroid. Found in cruciferous vegetables and soy. Most relevant in iodine deficiency; for most iodine-replete Hashimoto’s patients, cooked cruciferous vegetables in normal portions are not a meaningful concern.
MTHFR
A gene (and the enzyme it codes for) involved in methylation — converting folate into its active form (L-methylfolate). Reduced-function MTHFR variants are common and mean synthetic folic acid may not convert efficiently. More common in people with autoimmune thyroid disease. Reason Thyrolume uses L-methylfolate and methylcobalamin (active B12) rather than synthetic forms.