A synthetic thyroid hormone taken as a daily tablet. In genuine thyroid failure it is one of medicine's most reliable treatments. For mild, borderline thyroid signals it shows little or no benefit, while pushing levels too far raises risks to bone, heart rhythm, and lifespan. (Full Review)

Protocol

Standard Replacement

~1.6 μg/kg/day

Full replacement in overt deficiency, based on ideal (lean) body weight, as a single daily oral dose, titrated to a normal control signal (T4-only, once-daily consensus standard).

Conservative Start

25–50 μg/day

For subclinical elevation or older/at-risk patients; titrate slowly, or defer when the control signal is below ~10 units and symptoms are absent.

Best Time of Day

Morning, empty stomach

30–60 minutes before food. Bedtime dosing (at least 3–4 hours after the last meal) is a well-supported alternative that can improve absorption for some.

Time to effect

Symptom Improvement

Weeks to months

In genuine deficiency, symptom improvement typically unfolds over several weeks to a few months.

Blood Level Stabilization

4–6 weeks

Blood levels stabilize over 4–6 weeks after any change, reflecting the long ~6–7 day half-life.

Re-testing Interval

~6 weeks

The control signal should not be re-checked sooner than about 6 weeks after any dose change.

Benefits

High: Correction of overt hypothyroidism; lowering of LDL and total cholesterol

Medium: Improvement of fertility and pregnancy outcomes in defined subgroups

Low: Modest cardiac structural and vascular markers

Speculative: Symptomatic and cognitive benefit in "tissue-level" deficiency; metabolic and body-composition optimization

Contraindications

Untreated adrenal insufficiency (correct first)

Acute heart attack

Key Interactions

Absorption-reducing supplements and minerals (calcium, iron, magnesium, aluminum antacids)

Proton pump inhibitors and acid reducers (omeprazole, esomeprazole)

Bile-acid sequestrants and fiber (cholestyramine, colesevelam, psyllium)

Estrogen and oral contraceptives

Liver enzyme inducers (rifampin, carbamazepine, phenytoin)

Warfarin; insulin and other diabetes medications

Supplements with additive thyroid-raising effects (desiccated thyroid, T3/liothyronine, "thyroid support")

Risk & Side Effects

High: Bone loss from over-replacement; atrial fibrillation and cardiac strain from over-replacement

Medium: Symptoms of iatrogenic hyperthyroidism; lifelong medication dependence and over-treatment of mild cases

Low: Allergic and excipient reactions

Speculative: Long-term consequences of over-optimization in healthy individuals

Monitoring

Marker	Target	Why
TSH (thyroid-stimulating hormone)	~1.0–2.5 mIU/L (up to ~4.0–6.0 in adults over 70)	Primary gauge of whole-body thyroid sufficiency and dose adequacy
Free T4 (free thyroxine)	Mid-to-upper reference range	Confirms adequate storage-hormone level from the dose
Free T3 (free triiodothyronine)	Mid reference range	Reflects the active hormone tissues actually use
Thyroid peroxidase antibodies (TPO-Ab)	Negative / low	Identifies autoimmune (Hashimoto's) cause and likely permanence of deficiency
Reverse T3	Low-normal	Flags impaired conversion or non-thyroidal illness
Lipid panel (LDL, total cholesterol)	LDL per individual cardiovascular risk	Tracks a measurable metabolic benefit of correcting deficiency

Cadence: Re-check the control signal about 6 weeks after starting or any dose change, then at 6 months, then every 6–12 months once stable (every 4 weeks in the first half of pregnancy).

Qualitative Assessment

Energy levels and daytime fatigue
Cognitive clarity and freedom from "brain fog"
Cold tolerance and body temperature
Mood and sleep quality
Heart rate and palpitations (a rise suggests over-replacement)
Bowel regularity and stable body weight

Levothyroxine for Health & Longevity

Protocol

Benefits

Risk & Side Effects

Monitoring

Qualitative Assessment