For a few years now, I lack energy and strength. What could be wrong? When extensive lab work was done nothing untoward was found. How do I add T3 and 25-hydroxyvitamin D to my diet?

The short version: your labs are normal, but you are asking about adding T3 and vitamin D, which means you suspect the standard tests missed something real. The evidence here does not support self-prescribing T3 for fatigue, and vitamin D is worth checking with a blood test first.

You are describing a common and frustrating situation. Extensive lab work came back normal, but you still feel drained. That does not mean nothing is wrong. Standard panels miss a lot: they do not measure free T3, reverse T3, or look at how your cells use thyroid hormone. They also do not assess vitamin D status unless you ask for it. The evidence here is about how to raise those levels once you know they are low, not about whether raising them will fix your fatigue.

On T3: the evidence here is thin and does not support self-dosing. One retrospective chart review of 159 people with bipolar disorder found that adding high-dose T3 (mean 90 mcg daily) led to improvement in 84% of patients, but that was an uncontrolled study in a specific psychiatric population, not people with general fatigue and normal labs [9]. Another paper from 2005 describes a single case of a hypothyroid patient whose fatigue improved when liothyronine (T3) was added to levothyroxine (T4) [1]. That is a case report, not a trial. There is no randomized controlled trial here testing T3 supplementation in people with normal thyroid labs and fatigue. Adding T3 without a doctor and without knowing your current free T3, reverse T3, and TSH is risky. It can suppress your own thyroid function, cause heart palpitations, anxiety, and bone loss over time [9].

On vitamin D: the evidence is much stronger, but it is about correcting a known deficiency, not about treating fatigue in people with normal levels. Multiple studies show that vitamin D3 (cholecalciferol) raises serum 25-hydroxyvitamin D more effectively than vitamin D2 [2][3]. A 2015 randomized trial in 150 deficient adults found a clear dose-response: a loading dose of 200,000 IU followed by 100,000 IU monthly raised 25(OH)D by about 20 ng/mL over 12 weeks, with no adverse events [8]. Daily dosing of 1200 IU also works, raising levels from about 69 to 96 nmol/L in 3 months [6]. But the evidence here does not test whether raising vitamin D in someone with normal levels improves energy. One study in older women found that vitamin D3 increased muscle fiber size and vitamin D receptor concentration in muscle cells, which could theoretically help strength [7]. Another study in postpartum women found that supplementation raised vitamin D levels and improved pelvic floor muscle basal tone, but did not improve maximum contraction strength [5]. The link between vitamin D and fatigue in people with normal levels is not established by this evidence.

Here is what the evidence says about dosing:

My call: do not add T3 on your own. Get a full thyroid panel first, including free T3, reverse T3, and antibodies. For vitamin D, get your 25-hydroxyvitamin D level tested. If it is below 30 ng/mL, supplementing with vitamin D3 at 2000 to 5000 IU daily is reasonable and well-supported by the evidence here. If your level is already normal, adding more is unlikely to fix your energy. The real answer to your fatigue may lie outside what these two supplements address.

Confidence: moderate for the vitamin D dosing advice, low for any claim that either supplement will fix your fatigue.