Thyroid Labs and Levothyroxine Titration: TSH Patterns, Starting Doses and Recheck Timing

Thyroid questions on the FNP and AGNP exams start with a TSH and end with a dose and a date. Read the TSH first, add free T4 to name the pattern, and treat overt primary hypothyroidism with levothyroxine. Healthy younger adults can start the full replacement dose of about 1.6 mcg/kg a day; patients with known coronary artery disease start at 12.5 to 25 mcg a day; doses are adjusted by 12.5 to 25 mcg, and TSH is rechecked 4 to 6 weeks after each change per the 2014 ATA guideline (the current levothyroxine label says 6 to 8 weeks).

The rest of this article covers pattern recognition, the pitfalls that create false patterns, subclinical disease, starting doses by patient, administration, titration, special populations, the first steps for a low TSH, and the exam traps.

TSH first, then free T4

TSH is the most sensitive single test for primary thyroid disease because small changes in free T4 produce large, log-scale changes in TSH. The 2014 ATA hypothyroidism guideline uses a TSH within the reference range, roughly 0.4 to 4.0 mIU/L, as the therapeutic target for most treated patients, and the 2016 ATA hyperthyroidism guideline calls TSH the initial screening test for suspected thyrotoxicosis, adding free T4 and total T3 when thyrotoxicosis is strongly suspected.

The exception is central (secondary or tertiary) hypothyroidism. With pituitary or hypothalamic disease the TSH may be low or inappropriately normal, so free T4 is the test that diagnoses and monitors it, and the levothyroxine label directs titration to a free T4 in the upper half of the normal range.

Reading the patterns

False patterns to rule out first

• Biotin: high-dose biotin can produce a falsely low TSH and falsely high free T4 on streptavidin-biotin assays. Stop biotin at least 2 days before testing.
• Nonthyroidal illness: acute illness can lower TSH, and recovery can transiently raise it. Thyroid tests in hospitalized patients are interpreted cautiously and repeated after recovery.
• Drugs: glucocorticoids and dopamine suppress TSH; amiodarone and lithium cause hypothyroidism, and amiodarone can also cause thyrotoxicosis.
• Estrogen and pregnancy: raise thyroxine-binding globulin, which affects total T4 and increases levothyroxine requirements.
• Recovery after hyperthyroidism treatment: TSH can stay suppressed for a month or longer after thyroid hormone levels normalize, so free T4 and T3 guide early decisions.

Subclinical hypothyroidism

Subclinical hypothyroidism is a high TSH with a normal free T4. The 2014 ATA treatment guideline deliberately leaves adult subclinical disease to earlier reviews; the AACE/ATA 2012 hypothyroidism guideline is the reference here. Its framework:

• Confirm with repeat testing, because transient elevations are common, especially after illness.
• TSH 10 mIU/L or higher: treatment is generally recommended.
• TSH above the upper limit but below 10: individualize. Factors that favor treatment include symptoms of hypothyroidism, positive TPO antibodies, atherosclerotic cardiovascular disease or heart failure, and pregnancy or plans for pregnancy.
• Older adults: TSH rises with age in people without thyroid disease. The ATA cites a 97.5th percentile of 7.5 mIU/L in healthy elderly people, so a mild elevation in an 85-year-old is often normal for age.

A starting dose for mild disease is smaller than full replacement: the ATA notes that lower doses such as 25 to 50 mcg are typically sufficient when TSH is below 10 or the patient has subclinical disease.

Starting levothyroxine

The 2014 ATA guideline allows either full replacement or partial replacement with gradual increases, with TSH as the guide.

The label adds that doses above 200 mcg a day are seldom required. An inadequate response at more than 300 mcg a day is rare and suggests nonadherence, malabsorption or a drug interaction.

Administration and interactions

• Take once daily on an empty stomach, 30 to 60 minutes before breakfast (label). The ATA lists 60 minutes before breakfast as the most consistent option and bedtime, at least 3 hours after the last meal, as an acceptable alternative.
• Separate levothyroxine by at least 4 hours from drugs that bind it or reduce absorption, such as calcium carbonate, iron, bile acid sequestrants and phosphate binders.
• Coffee, proton pump inhibitors and malabsorptive conditions (celiac disease, atrophic gastritis, bariatric surgery) reduce absorption.
• Keep the patient on the same product when possible. The ATA recommends brand levothyroxine or maintaining the same generic preparation, and rechecking TSH after any switch.
• Peak effect of a dose change takes 4 to 6 weeks, which is why TSH is not rechecked sooner.

Titration and recheck timing

Targets: the ATA uses roughly 0.5 to 3.5 or 4 mIU/L for most adults and considers 4 to 6 mIU/L reasonable for adults older than 70 to 80. Overtreatment is common and harmful: TSH below 0.1 mIU/L is linked to atrial fibrillation in older adults and to bone loss and fractures in postmenopausal women.

Special populations

• Pregnancy: requirements rise early in the first trimester. For preexisting hypothyroidism, the label directs checking TSH and free T4 as soon as pregnancy is confirmed, increasing the dose by 12.5 to 25 mcg a day when TSH is above the trimester-specific range, monitoring TSH every 4 weeks until stable, returning to the prepregnancy dose after delivery, and checking TSH 4 to 8 weeks postpartum. Women planning pregnancy should know to call as soon as a test is positive.
• Estrogen therapy: raises the dose requirement; recheck TSH after starting it.
• Older adults: need lower doses, are more susceptible to overtreatment and may need help with daily dosing.
• Persistent symptoms with a normal TSH: the ATA found no consistent evidence that combination levothyroxine and liothyronine or thyroid extract is superior to levothyroxine alone, and it warns against overtreatment, so the next step is to look for other causes of the symptoms.

When the TSH is low: first steps

Primary care manages the first steps of thyrotoxicosis under the 2016 ATA guideline:

• Beta blockade is recommended for all patients with symptomatic thyrotoxicosis, especially older adults and those with a resting heart rate above 90 beats per minute or coexisting cardiovascular disease.
• Find the cause. If Graves disease is not clinically obvious, the ATA recommends TSH receptor antibody measurement, a radioactive iodine uptake, or thyroid blood flow on ultrasound. A scan is obtained when a toxic adenoma or multinodular goiter is suspected. High uptake points to Graves disease or toxic nodules; near-absent uptake points to thyroiditis, exogenous hormone or iodine exposure.
• Antithyroid drugs: methimazole is used in virtually every patient who chooses drug therapy for Graves disease, except in the first trimester of pregnancy (propylthiouracil is preferred), in thyroid storm, and with minor reactions to methimazole. Obtain a baseline CBC with differential and a liver profile before starting.
• Subclinical hyperthyroidism: when TSH is persistently below 0.1, treatment is recommended for adults 65 or older, those with cardiac risk factors, heart disease or osteoporosis, postmenopausal women not on estrogen or bisphosphonates, and anyone with hyperthyroid symptoms.

Referral and admission triggers

• Myxedema coma (hypothermia, altered mental status, hypoventilation) or thyroid storm: emergency admission.
• Suspected central hypothyroidism or pituitary disease: endocrinology and imaging.
• Graves disease needing definitive therapy, Graves orbitopathy or pregnancy with thyrotoxicosis: endocrinology.
• A palpable nodule or goiter with compressive symptoms: ultrasound and referral as indicated.
• TSH that will not normalize despite high doses after adherence and absorption are addressed: endocrinology.

Patient education

• Take levothyroxine at the same time daily on an empty stomach, with water, and keep calcium, iron and antacids at least 4 hours away.
• Expect symptoms to improve over weeks, not days; the first TSH check is at about 6 weeks.
• Report palpitations, chest pain, tremor or insomnia, which can signal too high a dose.
• Treatment of overt hypothyroidism is usually lifelong.
• Tell every provider and pharmacist about levothyroxine, and stop biotin 2 days before blood tests.

How the exam tests this

• Full dose in the wrong patient. 1.6 mcg/kg is correct for a healthy 35-year-old and wrong for a 74-year-old with angina.
• Rechecking too soon. TSH at 2 weeks after a dose change is a distractor; the expected interval is 4 to 8 weeks.
• Raising the dose before checking the basics. A rising TSH after the patient started calcium or iron calls for separating the doses first.
• Treating one mildly abnormal TSH. Subclinical hypothyroidism is confirmed before treatment, and a TSH of 6.5 in an 88-year-old may be normal for age.
• Dosing central hypothyroidism by TSH. Free T4 guides therapy.
• Using liothyronine or desiccated thyroid as first-line. Levothyroxine is the standard.
• Missing biotin interference in a patient with a thyrotoxic pattern and no symptoms.

Practice question

A 73-year-old man with stable angina and hypertension reports 4 months of fatigue, constipation and cold intolerance. TSH is 18.2 mIU/L and free T4 is low. He weighs 82 kg. He takes aspirin, atorvastatin, metoprolol and amlodipine. Which of the following is the most appropriate initial plan?

• A. Levothyroxine 125 mcg daily, recheck TSH in 6 weeks
• B. Levothyroxine 25 mcg daily, recheck TSH in 6 weeks and increase gradually
• C. Liothyronine 25 mcg daily, recheck free T3 in 2 weeks
• D. Repeat TSH and free T4 in 6 months before treating

Answer: B. Overt hypothyroidism in a patient with known coronary artery disease is treated with a low starting dose of 12.5 to 25 mcg a day, increased gradually by symptoms and TSH, with TSH rechecked 4 to 8 weeks after each change. Option A is the weight-based full replacement dose (about 1.6 mcg/kg), which risks provoking ischemia in a patient with angina. Liothyronine (C) has a short half-life and is not first-line, and a 2-week recheck is too early. Delaying treatment (D) is reasonable only for borderline subclinical results, not for a TSH of 18 with low free T4 and symptoms.

Key points

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