Hypothyroidism in Primary Care: Fast Dx → Confident Treatment

High TSH, low free T4, and a patient who feels run down. The diagnosis is the easy part. The questions that slow you down are dose, timing, and how often to recheck.

Here's how to start levothyroxine and titrate with confidence.

What's the lab pattern and the goal?

Overt hypothyroidism looks like:

• TSH: high
• Free T4: low
• T3: normal or low, usually not needed to make the diagnosis

Your three treatment goals are simple: reverse the symptoms, normalize the TSH (and free T4), and shrink a goiter if one is present. If you want a refresher on reading the labs first, see how to read thyroid labs fast.

Why levothyroxine first?

Levothyroxine (T4) is the first-line treatment. It's physiologic, has a stable half-life of about 7 days, and converts to T3 in the tissues predictably. That stability is exactly what you want for steady dosing.

Common brands are Synthroid, Levoxyl, and Tirosint (capsules or solution, often not covered). Tablets commonly covered come in 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, and 200 mcg.

How patients should take it

Teach this every time. It's where most "my dose isn't working" problems actually come from:

• Empty stomach, first thing in the morning, with water, 30 to 60 minutes before food or coffee.
• Separate from iron, calcium, magnesium, multivitamins, bile-acid resins, sucralfate, and PPIs by at least 4 hours.
• Stay consistent on the same brand or generic manufacturer when possible, and document the product.

How do I pick a starting dose?

Pick one approach and stay consistent:

• Healthy, under 60, no CAD: about 1.6 mcg/kg/day on actual body weight, or start 50 to 75 mcg daily and titrate.
• Older adults or CAD risk: 12.5 to 25 mcg daily, and go slow.
• Subclinical hypothyroidism (high TSH, normal free T4): shared decision-making. If you're treating, often start 25 to 50 mcg. Treat when TSH is 10 or higher, or with symptoms, positive TPO, goiter, or pregnancy plans.
• Pregnancy with known hypothyroidism: increase the total weekly dose by about 20 to 30% right away (roughly 2 extra tablets per week) and check TSH every 4 weeks in the first half of pregnancy.

How do I monitor and titrate?

• Recheck TSH (with or without free T4) 6 to 8 weeks after any dose change.
• Adjust by 12.5 to 25 mcg to the next tablet strength, for example 50 to 62.5 to 75 to 88 mcg.
• Target the lab's normal range. Many clinicians aim mid-normal, around 0.5 to 2.5, once symptoms resolve.
• Once stable, recheck every 6 to 12 months, or sooner with new symptoms, pregnancy, or new meds.

The one rule that prevents most confusion: TSH lags. Don't recheck earlier than 6 weeks after a change.

When would I consider liothyronine (T3)?

Liothyronine (Cytomel) is an adjunct, not a starting point.

• Who: select patients still symptomatic despite a normalized TSH and free T4, after you've ruled out other causes like poor sleep, anemia, depression, and OSA.
• How: small, split dosing such as 5 mcg in the morning and 5 mcg midday, titrate by 5 mcg, and monitor TSH, free T4, and total T3.
• Cautions: short half-life means peaks, with arrhythmia and angina risk. Avoid in significant CAD or older, frail patients.
• Trial: 3 to 6 months. Stop if there's no clear benefit.

What about desiccated thyroid?

Desiccated products like Armour and NP Thyroid combine T4 and T3.

• Not preferred to start, given variable T3 content, non-physiologic ratios, and more complex monitoring.
• If a patient strongly prefers it and is stable, educate on the risks and monitor carefully. Consider converting to levothyroxine when feasible.

Practical gotchas and fixes

• Biotin can falsely lower TSH and raise T4/T3. Have patients hold it 48 hours before labs.
• Iron and calcium bind levothyroxine. Separate by at least 4 hours.
• New meds like amiodarone, lithium, PPIs, sertraline, and antiepileptics can change dose needs.
• Weight change, pregnancy, and menopause all warrant a dose reassessment.
• Goiter regression is slow. Counsel patients to expect months, not weeks.

Frequently asked questions

How fast should patients feel better?
Energy, skin, and bowel changes can improve within 2 to 6 weeks. Weight and hair changes take longer.

Should I treat subclinical hypothyroidism?
It's reasonable if TSH is 10 or higher, TPO is positive, there's a goiter, the patient is symptomatic, or pregnancy is planned. Otherwise recheck in about 3 months before deciding.

How often do I recheck once stable?
Every 6 to 12 months, or sooner with new symptoms, pregnancy, or a new interacting medication.

Why isn't the dose working?
Most often it's timing or an interacting supplement. Confirm the patient takes it on an empty stomach and separates iron, calcium, and PPIs by at least 4 hours.

For the flip side, see hyperthyroidism first steps. Want the dosing on one page at your desk? The Clinical Desk Reference is a $37 quick-reference. For the full system on working through any visit, the Primary Care Clinical Mastery Program is AANP-accredited.

Education only. Use clinical judgment and your local guidelines.

Written by Allison Sowders, MSN, APRN, FNP-BC, a practicing primary care nurse practitioner and founder of Nurse Practitioner Mentor. Reviewed July 2026.