Can I Take Glycine with Cytomel (Liothyronine)?

What the Interaction Evidence Actually Shows

Glycine and liothyronine (the synthetic T3 thyroid hormone sold as Cytomel) do not share a documented pharmacokinetic interaction. No published trial has shown that glycine alters the absorption, distribution, metabolism, or excretion of liothyronine. The interaction question is reasonable because liothyronine is famously sensitive to timing and co-ingestion, but glycine's mechanism sits well outside the transporters and enzymes that handle thyroid hormones.

Why Liothyronine Is Timing-Sensitive

Liothyronine is absorbed in the small intestine via monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptides. Calcium carbonate, iron salts, cholestyramine, and high-fiber foods can reduce absorption by 20 to 40% when taken simultaneously [1]. Amino acids like glycine are not known to compete for these transporters, so the theoretical basis for a clinically meaningful absorption interaction is weak.

Glycine's Mechanism of Action

Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem, binding glycine receptors (GlyR), and also serves as a substrate for collagen, glutathione, and heme synthesis [2]. Its sedative and glycemic effects are largely receptor-mediated and metabolic, not transporter-mediated. That mechanistic separation is the main reason no interaction signal has appeared in pharmacovigilance databases for this pair.

Where Overlap Is Theoretically Possible

Both glycine and T3 influence insulin sensitivity and glucose metabolism. Liothyronine increases basal metabolic rate and glucose uptake in peripheral tissues [3]. Glycine improves insulin-stimulated glucose disposal through glycine receptor activation in pancreatic beta cells and skeletal muscle [4]. The two effects are additive in direction, not antagonistic, which is generally favorable but does warrant glucose monitoring in patients with diabetes or prediabetes.

Pharmacokinetic Profile: Liothyronine vs. Glycine

Understanding the absorption timelines of each compound explains why separation is recommended even when no direct interaction exists.

Liothyronine Pharmacokinetics

Liothyronine has a half-life of roughly 1 day (range 0.75 to 2 days), reaches peak serum concentration within 2 to 4 hours of an oral dose, and is nearly 100% bioavailable when taken fasted [5]. The 2012 American Thyroid Association (ATA) guidelines state: "Levothyroxine and other thyroid preparations should be taken consistently at the same time each day, preferably on an empty stomach, 30 to 60 minutes before breakfast" [6]. While that sentence refers to levothyroxine, the same pharmacokinetic logic applies to liothyronine, and most endocrinologists extend the same precautionary window to T3.

Glycine Pharmacokinetics

Oral glycine is rapidly absorbed from the proximal small intestine, peaks in plasma within 30 to 60 minutes, and is cleared with a half-life of roughly 12 to 17 minutes at typical supplemental doses [7]. At 3 g, plasma glycine roughly doubles from a fasting baseline. The rapid clearance means that if you take glycine 60 minutes after your liothyronine dose, peak glycine exposure will have passed before the two compounds overlap meaningfully in the gut.

Practical Dose-Separation Recommendation

Take liothyronine first on an empty stomach. Wait 30 to 60 minutes before eating, drinking anything other than water, or taking any supplement including glycine. This window is conservative but is supported by the ATA's own timing guidance for thyroid hormone preparations [6].

Glycine's Sleep Effects and Why They Matter for Thyroid Patients

Thyroid dysfunction, both hypo- and hyperthyroid states, commonly disrupts sleep architecture. Glycine supplementation at 3 g before bed has been shown in a randomized, double-blind, placebo-controlled crossover trial (N=11) to improve subjective sleep quality and reduce sleep-onset latency through core body temperature reduction mediated by peripheral vasodilation [8]. This is a separate mechanism from T3's thermogenic effect.

The Temperature Mechanism

Glycine lowers core body temperature by increasing peripheral blood flow, an action confirmed by polysomnography and skin temperature measurement in the Bannai et al. 2012 trial [8]. Liothyronine, when dosed correctly, does not produce significant hyperthermia in euthyroid-range patients. In overtreated (iatrogenic hyperthyroid) patients, however, elevated T3 can independently raise core temperature and worsen sleep. Combining glycine in that scenario may attenuate one symptom while the underlying over-treatment goes unaddressed.

Clinical Implication

If you are using glycine specifically for sleep and also take liothyronine, persistent poor sleep despite glycine supplementation could indicate over- or under-replacement of T3 rather than a glycine failure. TSH outside the 0.5 to 2.5 mIU/L range that most endocrinologists target for symptomatic patients warrants dose adjustment, not simply a higher glycine dose.

Glycine, Insulin Sensitivity, and Thyroid Hormone Interactions

This is the pharmacodynamic area most worth monitoring. Both compounds influence glucose metabolism, and their combined effect on insulin sensitivity has not been formally studied in humans.

Glycine and Glycemic Control

A randomized controlled trial by Cruz et al. (N=74, 3 months) found that 5 g/day of glycine reduced HbA1c by 0.5% in patients with metabolic syndrome, with a statistically significant effect at P<0.01 [4]. Serum insulin and HOMA-IR also improved. The proposed mechanism involves glycine receptor activation stimulating glucagon-like peptide-1 (GLP-1) secretion and direct insulin secretagogue effects in beta cells.

Liothyronine and Glucose Metabolism

Thyroid hormone receptors (TR-alpha and TR-beta) regulate hepatic glucose output and peripheral glucose uptake. Adequate T3 is required for normal insulin sensitivity; hypothyroid patients often show elevated fasting glucose and insulin resistance [3]. Replacing T3 to euthyroid levels generally improves insulin sensitivity.

Combined Effect in Patients With Diabetes

The additive insulin-sensitizing effects of glycine and correctly dosed liothyronine should theoretically benefit patients with type 2 diabetes or metabolic syndrome. Patients using insulin or sulfonylureas, however, should monitor fasting glucose more frequently when starting glycine, since an additive glucose-lowering effect could increase hypoglycemia risk. The American Diabetes Association 2024 Standards of Care recommend glucose self-monitoring adjustments whenever a new agent with glycemic activity is introduced [9].

Collagen Synthesis, Glycine, and Thyroid Hormone

Glycine is the most abundant amino acid in collagen, making up roughly one-third of its residues. Hypothyroidism independently impairs collagen synthesis and causes myxedema, a glycosaminoglycan and collagen-related tissue change [10]. Once liothyronine restores euthyroid status, collagen turnover normalizes. Supplementing glycine during the repletion phase is a common patient practice, and no evidence suggests harm from this combination.

Relevant Dose for Collagen Support

Studies on collagen synthesis typically use 5 to 15 g glycine per day from combined food and supplement sources [11]. A standard collagen peptide supplement providing 10 g protein delivers approximately 2 to 3 g glycine. Adding a separate 3 to 5 g glycine supplement brings total daily intake to 5 to 8 g, which remains within the range studied without adverse effects in healthy adults [2].

Who Should Be More Cautious

Most people taking Cytomel (liothyronine) can add glycine without concern beyond timing. Specific populations deserve a closer look.

Patients With Renal Impairment

Glycine is cleared renally and hepatically. In advanced chronic kidney disease (eGFR <30 mL/min/1.73m²), glycine accumulation is possible and has been associated with neurological symptoms at high intravenous doses (above 1 g/kg) in urological procedures [12]. Oral supplemental doses of 3 to 5 g/day are far below that threshold, but patients with significant renal impairment should discuss supplementation with their nephrologist.

Patients With Seizure Disorders

Glycine acts as a co-agonist at NMDA receptors in addition to its inhibitory GlyR activity. High-dose oral glycine (above 0.8 g/kg/day) has been used therapeutically in schizophrenia trials, and at those doses some patients reported worsening symptoms [13]. Standard sleep or collagen doses of 3 to 5 g/day are unlikely to reach CNS-active concentrations, but patients on antiepileptic drugs should confirm compatibility with their neurologist.

Pregnant Patients on Liothyronine

Glycine requirements increase during pregnancy because fetal collagen and glutathione synthesis create a conditional deficiency [14]. Liothyronine doses also typically require upward adjustment in the first trimester. Neither of these facts creates a new interaction, but both represent independent reasons to work closely with an obstetrician and endocrinologist during pregnancy.

Monitoring Plan When Taking Both

The following monitoring framework applies to patients combining glycine supplementation with liothyronine therapy. It is not a substitute for individualized clinical judgment.

Thyroid Function

• Check TSH and free T3 at baseline before adding glycine.
• Recheck TSH at 6 weeks if you changed your liothyronine dose at the same time.
• Routine TSH monitoring every 6 to 12 months once stable, per ATA guidance [6].

Glucose and Metabolic Markers

• Fasting glucose at baseline for any patient adding glycine.
• If you have type 2 diabetes, prediabetes, or use insulin or a sulfonylurea, check fasting glucose weekly for 4 weeks after starting glycine at therapeutic doses (3 to 5 g/day).
• HbA1c at 3 months if metabolic syndrome is present.

Sleep Quality

• Keep a brief sleep log for 2 weeks when starting glycine at bedtime (3 g, 30 to 60 minutes before sleep).
• Persistent insomnia despite glycine supplementation should prompt a TSH check to rule out over-replacement of T3.

Symptom Awareness

Report new palpitations, sweating, tremor, or diarrhea to your prescriber. These symptoms suggest liothyronine over-replacement and are not caused by glycine at standard doses, but the combination can make distinguishing the source confusing without lab data.

Practical Dosing and Timing Summary

The simplest approach:

1. Take liothyronine (Cytomel) on an empty stomach, 30 to 60 minutes before your first meal or any supplement.
2. Take glycine with breakfast, at another meal, or at bedtime (3 g for sleep; 5 g for collagen support), well after the liothyronine absorption window has closed.
3. Do not take glycine within 30 minutes of your liothyronine dose.
4. If you use split T3 dosing (a common practice for patients on T3-only or T3/T4 combination therapy), apply the same 30-minute rule to each T3 dose.

Split T3 dosing is used by some clinicians to blunt the mid-morning peak in free T3 that occurs with once-daily liothyronine [15]. A 2019 pharmacokinetic analysis confirmed that twice-daily liothyronine produces a flatter free T3 curve than once-daily dosing, though no outcomes trial has shown symptom superiority at the population level [15].

What Clinicians Say About Supplement Timing with Liothyronine

The ATA 2012 guidelines for hypothyroidism management note: "Patients should be instructed to maintain consistent timing, avoid co-ingestion with foods or supplements known to impair absorption, and report any new symptoms that might suggest over- or under-replacement" [6]. That framing, centered on consistency and symptom reporting, applies to glycine just as it does to any other supplement.

Endocrinologists who prescribe combination T3/T4 therapy routinely advise patients to separate all oral supplements from thyroid hormone doses by at least 30 minutes. Glycine's rapid gastric absorption and lack of known transporter interference mean the risk is low, but the standard precautionary guidance still applies.