T4 (Thyroxine)
The thyroid's main product. T4 is the predominant hormone made by the thyroid gland—about 80-90% of output. It's a prohormone, a reservoir that tissues convert to active T3 as needed. Think of T4 as the stable storage form, released slowly to maintain steady levels.
How T4 Is Made
TSH Stimulation
Pituitary releases TSH. TSH tells thyroid to make and release T4. Feedback loop.
Iodine Required
4 iodine atoms per T4 molecule. No iodine = no thyroid hormone. Deficiency still common.
Tyrosine Base
Amino acid backbone. Iodine attached to tyrosine. Thyroglobulin storage in gland.
TPO Enzyme
Thyroid peroxidase attaches iodine. TPO antibodies (Hashimoto's) block this. Iron needed.
Storage in Gland
Thyroid stores weeks of hormone. Released as needed. Buffer against fluctuations.
Protein Binding
99%+ bound to proteins in blood. Only free T4 is active. TBG main carrier.
T4 vs T3
T4 Characteristics
- Less potent: 3-4x weaker than T3
- Long half-life: ~7 days
- Stable levels: Doesn't fluctuate much
- Storage form: Reservoir for conversion
Why T4 Matters
- Stability: Provides steady hormone supply
- Local control: Tissues convert as needed
- Medication form: Levothyroxine is T4
- Testing: Free T4 key thyroid marker
Low T4 Causes
Hashimoto's
Autoimmune thyroid destruction. Most common cause of hypothyroidism. Progressive damage.
Iodine Deficiency
Can't make T4 without iodine. Still common globally. Goiter develops.
Pituitary Issues
Low TSH = low T4. Central hypothyroidism. Pituitary tumor, damage.
Thyroid Surgery/RAI
Removed or destroyed thyroid. Requires lifelong T4 replacement.
Nutrient Deficiencies
Iron, zinc, selenium, vitamin A needed. Tyrosine is building block.
Medications
Lithium, amiodarone affect thyroid. Some interfere with absorption.
T4 Replacement
Levothyroxine
Synthetic T4. Most prescribed thyroid med. Synthroid, Levoxyl, generics. Standard treatment.
Dosing
Usually 1.6mcg/kg. Adjusts based on TSH. Takes 6 weeks to stabilize after dose change.
Absorption Tips
Take on empty stomach. 30-60 min before food. Avoid calcium, iron, coffee near dose.
Brand Consistency
Potency varies slightly between brands. Stick with same manufacturer. Tirosint liquid form.
Still Symptomatic?
May not convert T4 to T3 well. Check free T3. May need T3 addition or desiccated thyroid.
Monitoring
TSH, Free T4, Free T3. Annual or after dose changes. More if symptomatic.
Understanding T4 Tests
Free T4 (fT4)
Unbound, available hormone. More accurate. Not affected by protein levels. Standard test.
Total T4
Bound + unbound. Affected by binding proteins. Pregnancy, estrogen increase it. Less useful.
High Free T4
Hyperthyroidism (if TSH low). Or taking too much medication. Adjust down if symptomatic.
Low Free T4
Hypothyroidism (if TSH high). Or central issue (if TSH low/normal). Needs treatment.